Thread: Does epoxy stick to silicone?

I have a lot of experience with silicone and epoxy. Be warned that silicones aint silicones. Epoxy may not stick the first time but that doesn't mean it will continue to not stick in subsequent applications. This is even more of an issue for softer grades of silicone. the softer the silicone is in a cured state usually means there are more and more additives being put into the mix. These different constituants can cause a reaction with the epoxy over time. The epoxy will eventually turn this type of silicone a different colour and make it more brittle, then the epoxy will begin to stick and you may as well throw it out. This may happen after just a few uses.

Be careful on your assumptions, i'de imagine silicone brushes are quite soft!

Originally Posted by jmk89

Hi Mik

These people seem to sell them. I wonder which nap would be best.

They have a 6mm or 12mm nap. Given that the best foam rollers would be less than 6mm ... I would say there's your answer.

I would expect a shorter nap still to be better. But might take a few tries by someone to see.

MIK

Ash233, thanks! That's exactly the type of info I was looking for.

Economically and environmentally, a silicone brush may still make sense if I can get a few dozen uses out of it, and if it spreads the epoxy well until it dies. It doesn't need to last forever, just until the break-even point. I guess there is no way to know how many uses I can get until I try.

But your talk of reactions with additives brings up my larger worry: will something in the brush ruin the epoxy? The silicone itself may not melt or dissolve, but one of those additives might. And then I've got who knows what mixed in with the epoxy on my boat. Will it soften the epoxy? Stain it? Cause it to break down over time?

I think I will contact the manufacturer and ask them if they have tested with epoxy. Even if they have, I might run a test or two myself before using one of these brushes on a critical part.

Originally Posted by Boatmik

...I would expect a shorter nap still to be better....

How about a roller with no nap at all? My artist friend spreads ink on his lino-cut blocks with a rubber roller, yielding a very thin, even coat. We're not using these rollers like paint rollers -- dipping them in a puddle. We're using them to even out a coating we've already spread on the surface with something else. Do we need any nap at all?

One of the problems with foam rollers, of course, is the bubbles they leave behind. Would a solid roller leave fewer?

Of course, a silicone roller would come with all the same caveats as a silicone brush: Will it taint the epoxy? Will it soften or harden over time? How may uses can we get before the hardened epoxy won't peel off anymore? Etc.

But, assuming those questions have satisfactory answers, would a silicone roller work to smooth epoxy and pop the bubbles?

Originally Posted by paulie

Ash233, thanks! That's exactly the type of info I was looking for.

Economically and environmentally, a silicone brush may still make sense if I can get a few dozen uses out of it, and if it spreads the epoxy well until it dies. It doesn't need to last forever, just until the break-even point. I guess there is no way to know how many uses I can get until I try.

But your talk of reactions with additives brings up my larger worry: will something in the brush ruin the epoxy? The silicone itself may not melt or dissolve, but one of those additives might. And then I've got who knows what mixed in with the epoxy on my boat. Will it soften the epoxy? Stain it? Cause it to break down over time?

I think I will contact the manufacturer and ask them if they have tested with epoxy. Even if they have, I might run a test or two myself before using one of these brushes on a critical part.

I think you'll have no problems or adverse effects with the epoxy paulie. I've never seen anything of that nature. The epoxy tends to damage the silicone not the other way around.

As for your roller question, no nap is probably a better thing! The type of rollers used in the fibreglassing industry are metal rollers with grooves machined into them to help consolidate the laminate and wet out the fabric.

heres an example though the ones I've seen are just straight grooves...

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They should last a while if you look after them properly.

As far as silicone affecting epoxy, it doesn't seem the usual compounds, groups and modifiers would do anything but remain in suspension in the liquid goo, becoming little more then frozen particulates in the cured matrix. The quite alkaline nature of epoxy could bother some silicon formulations and this is a removal technique, but the brushes and rollers in question are probably more closely related to acrylics then actual silicons. These formulations are probably acrylics with silicone modifiers. Since the word silicone is a much better marking tool then the word acrylic, they "punch up" the value of this modifier on the label.

As a rule, brushes are a very poor way of applying epoxy. With straight epoxy they tend to make a puddle where the brush first touches, then the edges get feathered as it's stroked. This produces a very uneven coating. Halving the bristles can help to a degree, but I've found this is only really effective with thickened epoxy, not neat batches. Unfortunately, some places just need a brush to get into, so keep them on hand, but learn to use other tools for the broad areas.

Rollers on the other hand, do make a much more uniform coating, though most tend to over stroke the goo in application. If used properly, you can get really good, even coatings with a roller. I use solvent tolerant rollers, which cost more, but don't fall apart, shed or other wise screw up a coating.

I've found the best method to apply uniform, even epoxy coatings (no tipping!), regardless of viscosity, are plastic bog applicators or better yet (with some practice) rubber squeegees. On raw wood, my intial goal is to seal the wood and get some penetration, which saves labor later. I'll literally mash a coating into the surface, scraping hard, forcing the goo into the grain. I use the angle and the flex of the blade to do this. I often use a putty or drywall knife for this reason. The result often looks like I barely touched the surface with goo. The pay off is the next coat, which goes on in a uniform layer. It's a pressure thing and you kind of feel your way through, trying to maintan a consistant pressure on the squeegee as you smear it around. Once you get the hang of this technique, you can apply this second coat nearly perfectly, except for some track marks at the end of the blade. You can half these marks by angling the blade like a snow plow and always pushing the excess goo to the next area you'll swipe with the squeegee. This leaves only one track mark, instead of two. The track marks are easily removed with a scraper once it's cured enough.

Lastly, on clear coat finishes, an easy way to get that mirror finish is to use Mylar and a roller. Apply your last coat of epoxy, then place Mylar over it and very genitally roll it into the surface of the wet goo. On compound curves, you'll have to work out diagonal patterns before hand, so you don't get puckers, but on plywood planked hulls, you can do whole sides, bottom and decks in one shot, assuming you have a big enough sheet of Mylar. I get it on rolls and print templates on the stuff, so I always have some around. A big piece of Mylar will last a life time, if you take care of it. Once the epoxy is good and cured, just peel the Mylar off the surface and presto, it magicly reveals a mirror like epoxy coating. If you have to scrape some lap marks, where multiple pieces of Mylar where applied, its a minimum amount of work. This trick is a huge time saver and can make anyone look like a real pro. I did a buddy's bathroom cabinet top with two sink basins in it, this way. I made the top from nice pieces of Douglas fir, edge glued, then very lightly stained. A did a couple of sealing coats the usual way, but the last coat was Mylar covered. He swears it's a piece of glass glued to the surface and it looks like it. All technique and a little practice, nothing fancy, just good goo procedures.

Those are some helpful tips. I'm definitely going to try the squeegee method next time. I wonder how big of an influence temperature has one the various methods. I can see a roller working a lot better when the epoxy is warmer, but when you're doing epoxy work only a few degrees above 0 it gets very thick and I can see spreading it with the squeegee working a lot better.

I thought I had a roll of drafting mylar but turns out it is actually cotton vellum. I can see that working extremely poorly as a substitute! That method does sounds useful though.

Naturally viscosity can be a big problem in cold weather, but the squeegee method works well, regardless of viscosity. In cold weather, which isn't sometime I see a lot of, I'll switch to a flimsy steel drywall or putty knife. I can apply more pressure to move the thicker goo. Trust me, once you get a handle on the squeegee, putty knife, plastic bog applicator technique, you'll never even think about a brush again, except in corners or other hard to reach locations where wasting a chip brush is the only way. With rollers, which also work well, I feel guilty about all the goo still trapped in the roller when I have to toss it in the can. With the squeegee method, there's no waste. I've probably thrown away gallons of epoxy in used rollers over the years. That cuts substantially into my beer money.

Nice info PAR ... as always. I should have followed my normal habit of when I haven't heard of something ... ask PAR!

Paul is right about rollers, but there are always corners and bits of glassing that need a brush.

One thing I thought of on rereading the above is most normal paint brushes are too soft for epoxy. Generally it makes sense to trim disposable (chip brushes) down to about an inch long. The result is the amount of epoxy is more controlled ... you can get more on the surface without adding so much to the brush.

With a smooth roller ... I don't think it makes sense. It is going to distribute epoxy pretty much like a sqeegee.

A normal thin foam epoxy roller cover is great for redistributing too much epoxy in one area to another dryer area when a squeegee just can't handle it as the epoxy gets distributed into a thinner layer.

The compressing rollers with the rings are really a very blunt tool - a way of getting air out of dense layups of chopped strand matt. I don't recommend them for the type of light fabric glassing we do because they potentially damage the glass and the thickness of glass we are generally using means a brush or squeegee can remove the air easily and simply.

Squeegees are BRILLIANT for a first coat onto ply or glass, but there are a couple of things to be aware of.

First is that the corners of the squeegee can scrape some of the previous layer off when you do the second or third coats ... so I often round them subtly just to remove the sharp corner.

Also if the second coat epoxy is poured on it can reliquify the coat underneath if it sits in pools for too long ... so good to get on with spreading.

The thin foam rollers are really good to control the amount of application and reduce the risk of distribution streaks from the epoxy flowing off the edge of the squeegee.

My general work practice is to use squeegee for the first coat. Then maybe to use the squeegee for quick distribution of the second coat (after the first one goes tacky of course) and follow up with the roller to finesse the distribution of the epoxy. Usually the final coat is almost all roller unless the surface is very horizontal.

Best wishes
Michael

Originally Posted by Andykane

...roller working a lot better... squeegee working a lot better.

Well I sure didn't make my thoughts very obvious there.

What I meant to say what I think the roller would be a lot more effective with warm epoxy than cold, wheras a squeegee will probably work better than anything for cold epoxy.

I have used a squeegee for coating glass and found it worked very well.

Yes!

I usually use a sqeegee for glasswork too .. but not usually when glass taping ... there I am more likely to use a brush. There are kindof different methods for taping and small areas compared to big area glassing. They are described in my FAQ and also in the plan appendices.

FAQ - Boat Building and Repair Methods - Plywood Epoxy Fibreglass Cedar Strip - Michael Storer Wooden Boat Plans
I do think the squeegee is less effective after the first two epoxy applications when glassing. After that you can use it for quick distribution of the epoxy, but it is very uneven for the relatively smaller quantities used for follow up coats .. so you end up rollling anyhow.

I haven't ever applied epoxy close to freezing but I generally choose brush, roller or sqeegee because of the nature or size of the job rather than to do with temperature.

One thing that may make lower temperatures easier is to use a lightbox to keep the resin and hardener a little warm before pumping it out to mix.

Best wishes
Michael

Originally Posted by Boatmik

I haven't ever applied epoxy close to freezing but I generally choose brush, roller or sqeegee because of the nature or size of the job rather than to do with temperature.

One thing that may make lower temperatures easier is to use a lightbox to keep the resin and hardener a little warm before pumping it out to mix.

The temperatures in the shop (Central Michigan) these days are in the low to mid-30s and I have been working on the frames, centerboard and rudder for my Pathfinder build.

A lightbox works well to keep the the resin and hardener warm prior to mixing. A 100W bulb in a 300mm cube maintains a temperature between 75°F and 80°F. However, the parts being epoxied are room temperature (cold) and the mixed epoxy quickly cools and becomes more difficult to work than when the wood and epoxy are both warm. I have not come up with a practical way to warm the parts to 75°F or 80°F. It is not currently practical to heat the space.

As for method of application, I use a spreader/squeegee for large surfaces, a 4" roller with ¼" nap for mid-sized pieces (the CB staves are 75mm wide x 1100mm long and the roller worked well on these) and cut-down chip brushes for tight spots (corners and the like).

Nearly all the parts (bottom, CB case, frames and stringers) needed to assemble the hull are done and I expect to begin assembly next week.

I use different size squeegees for the jobs at hand. For taping, I'll use a squeegee that's about 2/3's the width of the tape. This means I can wet out most of the tape in one pass, leaving two tracks of epoxy at each end. I then go back and push these tracks into the edges of the tape.

There are some tricks to using them, one Mik mentioned, which is to radius the ends, so you don't dig in. Another I use often is instead of pouring epoxy on the surface I'll dip the squeegee in the goo, then quickly wipe it on the location I want it. I can move large or small amounts this way.

On small parts, I tend to make a "dipping" box, which is just a cardboard tray I make, with shallow sides and covered in packaging tape or plastic sheeting. I smear a relatively thin amount of goo on the bottom then dip a part into it, such as a centerboard stave. I lift the stave up, smear the goo evenly on the side that just got dipped, with the excess dropping back into the shallow tray. I then rotate the part, repeating the process until all sides are coated, where it then is placed somewhere to kick off.

My idea of a squeegee is little more then an1/8" or 1/4" thick rubber strip cut off a 4" wide roll. I pull off a length I need, cut and sometimes even bother with a handle, but most times not. If the edge gets torn up, I just cut a new edge with a ruler and a blade. I have several different ones that I've made over the years, some have flexible handles, other stiff, while others none at all. With practice you'll leave brushes and rollers in the drawer most of the time.

To give you an example, I have a 24" long 1/4" rubber squeegee, that I use for large fairing jobs. It has a piece 3/4" PVC pipe on one edge, which is flexible, but stiff enough to do flat work too. I can apply a 24" wide, 8' long smear of fairing compound, in one single swipe of this baby. It conforms to curves and if you're careful about the pressure in use, a very even thickness of goo can be applied. A guy with a 6" putty knife will take the better part of 20 minutes to apply an even coat 24" wide by 8' long. I can do it in less than a minute. I find the speed difference is true, for wetting out fabrics too.

Another quick and easy squeegee that is OK for my flat panel boats is an offcut of ply taken from the manufactured edge of the sheet.

It is good enough in a pinch and I've used it many times. I just sand the edges a little round and put a tiny radius on the corners.

There are lots of ways to make or get a sqeegee. There are many that are a bit nicer to use than a ply offcut .. but almost nothing beats the convenience.

MIK