Thread: polyurethane finish still rough

There are a number of things that are not generaly understood about polly.

the most important is that if you expeect it to perform anything like a fine finish product you must thin it.....however if you thin it too far some products will go slightly milky in the dried finish..so stay under 10%.

While poly is green... not off hard.. it will chemiclay key in to the preceeding coat.....once it is properly off it will not.... most recommend recoating within 48 hours if not sanding...

The biggest revalationafter thinning is using a sanding sealer.....preferably the one from the same manufacturer with the same resin base... for example estipol sanding sealer under estipol top coat.
this stuff sands so much better than the top coat and can be safely sanded a little earlier.

work up the base for your finish with the sanding sealer...2 , 3 or however many coats you need to get a flat choked finish.

do not skip sanding steps and do not expect the next step to fix the faults of the previous step.....this goes for sanding and finishing.
if you havn't fixed a fault varnishing over it wont fix it.

There is a big temptation to over sand and sand far too fine....poly is a thick film finish.....

A rep told me and fine wood working did some tests a while ago.
There is no point sanding finer that P180 with thick film finishes either bare timber or between coats..I sand to P240 but only because it makes me feel better.
If you sand too fine particularly on fully cured coats you will run the rish of adhseion problems.
A proper job with no sanding errors is better than sanding finer.

you need to watch your temperature and humidity....if it is too hot or cold or too dry or humid the finish product will not work as smoothly.

you need to select a good brush and in good condition...I prefeer an oval cutter bcause it gives me a better overlap.

you need to have your brushing planned and you need to work at keeping that wet running edge....now people disagree and I have used diferent methods....and it will change depending on how fast the finish is going off.

but regardless get it down fast and smooth, make every stroke perfect if you can the film needs to be thick enough that it flows out before it starts going off.....if you can avoid gong over parts that have started to cure......you can do a go over... but the window is narrow and you run the risk of adding too much build and draging the finish.

unless you have a cleanroom you will never get a perfect finish off poly with eother brush or spray....it just takes soo long to go off there is so much opportunity for carp to land on it.

so
depending on your preference and how fussy you are

rub out with steel wool and wax

rub out with wet and dry ( wet) 800 to 1200 will be fine and do not over sand, just get rid of the lumps.....dry off the overcoat with wax.

rub out with a scotchbrite pad, dry then wax

rub out with a scotchbrite and wax.

supermarket scotchbrites are OK I supose.. I have used them.

but if you go to an automotive refinish supplier of a commercila cleaning supplier you can get them in grades.....green is similar to domestic scourers.....maroon is finner...... greau is finer still.

maroon I find good.

remember poly is not a fine finish product so you have to drag a good finish out of it kicking and screaming.

as fo the durability of wax.
as long as you use a wax bassed on hard waxes ( like ubeaut traditional wax) it wears just fine.

Oh I tend to stay away from the gloss product and the full gloss finish....a mat finish shows less flaws.

some will preferr a gloss product and buff back to a mat finish because some of the gloss products are harder than their satin brothers.

cheers

Thanks for the tips guys

In the end, I rubbed the table back with steel wool, then applied a wax. The finish feels pretty good, but with the tips i've been reading i think my next attempt will be better.
I suppose it's a learning process, albeit an enjoyable one.

Hickory - didn't realise that the poly had to be hard before reapplying. I think I followed the directions on the tin too closely - I should have taken the weather conditions into account more. This means that I should have waited a little longer, until the poly was hard, to apply second and third coats. Thanks for the advice!

John

Originally Posted by Johnny

In the end, I rubbed the table back with steel wool, then applied a wax. The finish feels pretty good, but with the tips i've been reading i think my next attempt will be better.

Got pics?

there are twp phases to the curing of poly.

the initial going off....lets call it the green stage... then the hardening phaze I supose we could call it.

when pollynis applied it will cure to what appears to be a set finish....then over a 24 to 36 hour period depending on temperature and humidity it will go off to a hard or fully cured state..

now

If you overcoat polly without sanding once it is in the fully cures state there will be serious adhesion problems with the second coat..... once fully cured one coat will not chemmicaly bond to the next........this is usulay not a problem with furniture as we generaly sand between coats..... but when people put this stuff on buildings this is a common finishing flaw.

now iff one coat is put over another before the first coat has cured out one coat will chemicaly key into the coat below.

I can see no problem at all putting one coat over another that has not fully cured out......if the first coat is tooo green you will find problems with sanding....if it is realy green the film will pull........if the film is a little less green you will have clogging problems with your abrasives.....
There is a point where the finish is sufficiently cured to sand well but still not be fully cured.
It is my view that this point will give a better bond between caots than letting the finish go off and fully harden.

the other problem is that if you are letting the finish fully harden you may be waiting 3 or more days between coats..

but this is the problem with single pack poly....it is a slow process.

If you are going to let the finish cure out fully between coats it is goung to take you a fortnight to put a finish on a piece.

If you take the minimum cure option you could do the same job in 3 or 4 days.

Personaly I do not believe there is any advantage to letting the finish fully cure out between coats.

cheers

Here are a few pics. Didn't know how best to take the photos so I put the table I made up onto the dinning table. It's only a small table, sits beside my couch to hold drinks, books etc.

Overall i'm pretty happy, but there's room for improvement. And thanks for the follow up comments, soundman. Helpful to know your thoughts on poly.

John

OK, I live in Florida, USA, and it's heading into summer, here. I'm not quite clear -- is the affect of the humidity levels (very high in general in this area and definitely right now) on the poly to increase the cure time? That sounds like what is being said, and seems to make sense. Assuming not-quite-jungles-of-Borneo humidity (particuarly since it's raining right now, and my workspace is covered and screened but outdoors), how long should I let the stuff cure?

Also, you suggest starting with a very clean surface. For future reference, what is the best mechanism/procedure for removing any remnant dust from the sanding process? I wiped the wood down with a damp cloth several times, but I'm not sure I was fanatical about it.

The wood itself is unspecified 3cm planking, probably pine I'd guess, not too soft, not too hard, substantial amounts of grain. It was purchased as much for its price as its strength -- I've found 3cm planks to cost no more than 1.5cm "boards", and you can make bookshelves up to 2m long without intervening support.

In my own case, I just sanded it with a finishing sander using 80 grit paper until it was satisfactorily smooth to the touch, then cleaned it as mentioned above. I notice after a single coat (applied with a brush, using MinWax satin poly, probably it's a fairly thin coat), the wood is now like fine sandpaper. It looks good, the minwax brought out the grain nicely, pretty much just like I wanted, I'd just prefer it to be smoother.

I'm assuming my next step is to sand it with 120 (does this really need to be done by hand or is a finishing sander acceptable at this stage?) and then re-apply the poly. 120 is the finest grit I currently have, do I need to use something more like 180 for the stage between coats 2 and 3? Or should I abandon a 3rd coat and use a wax in its place?

I'm not fanatical about the results, but I'd like to do a decent job and not much up the purpose of the poly. The wood itself is going to be part of a bookshelf. I'd like it to be at least fairly smooth to the touch.

Since I hadn't done this before, and wasn't expecting the sandpapery results I thought I'd check in somewhere with people who knew a lot more than I do, just to see what was suggested.

I'd suspect that the first coat is pretty much fully, hard cured despite the local rain for the last couple days, but that need not happen with subsequent coats, and it's also pretty thin. I used more of a "wet the wood" approach than any sort of thick first coat effort, on the presumption that the wood was going to soak in most of the first coat.

Any suggestions, comments, observations ("YOU IDIOT!!!" is even ok... especially if it includes explanation for that appelation ) would be appreciated

Thx.

--------------------------


Heeeey!!! Aussie, NZ, Canadian, Brit, Welsh flag smilies -- ok... but


Greece, France, Japanese and Swedish flags, even Hong Kong, but no US flag!?!?
What a gyp!!

Tyler,

Living in the South myself, I can say that the high humidity will indeed retard the curing process, though higher temps will speed it up.

A couple of points for your particular application:

The sandpapery feel you have is mostly from the grain being raised (and perhaps a few dust nibs). Particularly noticeable if you used water-based poly, though the first coat of almost anything will raise grain in pine.

You should have sanded to at least 180 grit before finishing. 80 grit leaves a rough surface and contributes to the grain problem. You should now be sanding back with 320 to 400 grit (or at least 240). 120 grit is still fairly rough and certainly not for sanding back finishes. If you are going to Lowes or Home Depot, you will find the finer grits in the paint department, not the tool department, which usually only carries up to 180 grit. Buy the gray wet and dry in the finer grits.

Give it a light sand. You only want to get rid of the "nubs" not all the finish. Sand a bit then feel with your hand. Your hand is a sensitive barometer. When smooth, apply another thin coat, let dry overnight, and I think you will find the finish much smoother. You can use your finish sander, but with a light touch. Go over the surface once, then stop and feel with your hand.

As for cleaning off sanding dust, wiping with a damp cloth was a mistake. That ensured the grain would be well raised. Wash down with a cloth dampened with mineral spirits or you can use a tack cloth if not using water-based poly. Do not, however, use a tack cloth under a water-based finish.

>Tyler,

>The sandpapery feel you have is mostly from the grain being raised
> (and perhaps a few dust nibs). Particularly noticeable if you used
> water-based poly, though the first coat of almost anything will raise
> grain in pine.

OK, that makes sense, and it was a standard minwax poly, I didn't even know they made water-based polys. But yeah, it wasn't water-based.

> You should have sanded to at least 180 grit before finishing. 80 grit
> leaves a rough surface and contributes to the grain problem. You
> should now be sanding back with 320 to 400 grit (or at least 240).
> 120 grit is still fairly rough and certainly not for sanding back finishes.
> If you are going to Lowes or Home Depot, you will find the finer grits
> in the paint department, not the tool department, which usually only
> carries up to 180 grit. Buy the gray wet and dry in the finer grits.

OK. Will do.

> Give it a light sand. You only want to get rid of the "nubs" not all the finish.
> Sand a bit then feel with your hand. Your hand is a sensitive barometer.
> When smooth, apply another thin coat, let dry overnight, and I think you
> will find the finish much smoother. You can use your finish sander, but
> with a light touch. Go over the surface once, then stop and feel with your
> hand.

> As for cleaning off sanding dust, wiping with a damp cloth was a mistake.
> That ensured the grain would be well raised.

OK, though I did allow it to dry fully (or as much as would occur in the humidity before proceeding), a good 12-24 hours. Did that still matter, just for future reference? I assume so, but it helps to understand where the problems lie.

> Wash down with a cloth dampened with mineral spirits or you can use a
> tack cloth if not using water-based poly. Do not, however, use a tack
> cloth under a water-based finish.

OK, any difference for this purpose between mineral spirits and something sold as "paint thinner" (right next to the "mineral spirits" but cheaper) for that purpose? I know paint thinner, while often used interchangeably is a somewhat wider-ranging term so it might be that there is a difference for that purpose.

Also, just out of curiosity -- what about the final result -- should I get some kind of wax or is the poly finish good by itself? Is there anything like buffing I should do? I'm using a satin finish so no gloss would be sought. And the surfaces in question won't be largely exposed to view (as on a tabletop)

Thanks.

Tyler,

It was the water in the damp cloth that raised the grain. Letting it dry was not the issue, getting it wet with water was.

Paint thinner should be OK for this purpose.

The poly finish will be fine by itself. Don't bother putting anything over it for your application.

Here is a bookshelf I built, stained, and finished in poly a few years ago. Still looks good.

Originally Posted by Honorary Bloke

Tyler,

It was the water in the damp cloth that raised the grain. Letting it dry was not the issue, getting it wet with water was.

Paint thinner should be OK for this purpose.

The poly finish will be fine by itself. Don't bother putting anything over it for your application.

Here is a bookshelf I built, stained, and finished in poly a few years ago. Still looks good.

Looks nice -- I'm not doing anything that complex, just a four-plank assembly designed to fit under a door-converted-to-desktop unit I made with 4x4s. 2 4' shelves, one of which will rest on the floor, and one about 15 inches above it. I wasn't originally going to get this fancy, I was going to leave the wood unfinished but decided to do it anyway.

I was originally going to just put a large plank underneath (I use the end of it as a desk surface -- I cantilevered it out for that purpose), but decided that my computer books would make more sense under the desk in question than whatever else I might place there (most other books are upstairs and not handy when working on the computer, so it seemed to make more sense. That also will increase stability even more by putting a lot of weight in the center. I did a fairly good guess on the cantilever, it doesn't tip easily, but I noticed if I put my full weight on it it does shift, so it'll be better with more weight somewhere.

One of the rules was "cheap" -- I'll probably have spent more on tools and externals (poly, sandpaper, etc.) than the stuff in the project itself.

Attached are two pix, the short, vertical board, and both size boards, adjacent
(The long is 4', the short is 28")

Two of each, they'll be put together in an inverted, squaretop "A",
with the short ones on the sides : |---|

No need for a "top" board, as the horizontal, solid-core door will be there.

I've got to get something cheap for the backer, I'm assuming some type of pressboard will do for now, I guess I'll paint it beige to match. I might replace it with something better, later but that will do for now, since it will be back, in the dark, with books in front of it.

You can see what I'm doing is far from complex. I'm thinking if getting a doweling jig and using dowels rather than screws, but that's not something I'm sure of, yet. I love to own tools and do things, even if I'm not getting fancy for lack of a permanent workspace.