Thread: Seeking advice re. slabs and epoxy?

Originally Posted by Mr Brush

I have a blackbutt slab, about 1m x 500mm x 35mm thick, which I have laboriously flattened/thinned out using a router sled and sanding to the point that I need to think about filling some of the surface cracks before final sanding. I'll be adding blackbutt boards across the ends (dominoed) to hopefully stop it bowing.

See photos of the whole slab and some close ups of detail - there is a lot of interesting fine detail on the surface I plan to have as the top, plus a few wider/deeper cracks which need filling.

I'm planning to use Luci-Clear epoxy, and I have black tint to go with this as well. Final finish for the top will be some kind of hardwax oil like PolyX to give a satin finish.

I just can't decide whether to use clear epoxy or tint it black????

If I go with black, a lot of the interesting detail in the gum lines and features may disappear into large areas of black, while clear would allow this to be seen. I'm guessing the epoxy will also show through shinier than the general finish on the slab?

I gather that the technique is to scrub all the cracks out with a toothbrush + acetone, blow them out with compressor, work a thin layer of epoxy into the cracks/voids with a brush to seal the surfaces, then do the final pour of black/tinted epoxy to overfill the cracks slightly?

Any tips/advice much appreciated, as I haven't tried any of this before

Your end boards (breadboard ends) won't work. They need to slide or you will either break the joint or split the top. Google bread board ends and have a look at how they're made and assembled.


I'm all for hiding defects in plain site by adding a tinted epoxy or using a filler that is completely different to the defect to actually accentuate them. As such there would be no need to scrub the defects out, just fill them.

Yes, they will have a much different sheen and surface than the surrounding wood as the epoxy is a crystalline filler that will sand up much and take a finish differently than the surrounding wood will. The difference will be very noticeable, especially if back light by a window. Whether it looks bad or good is up to the viewer. The only way to counter the difference is fill the pours in the wood and to lay a finish over the entire surface.

Thanks for the suggestions.

I think I've got the ends sorted. Only the centreline domino is the tight fit on both pieces, next ones out (about 60mm away) are tight fit in the slab, but loose fit in the end piece. The outermost domino points are a tight fit in the slab, but a slotted kind of drawbore joint using a small dowel and loose domino hole in the end piece. The slab can move about 3mm in either direction at the outer edges, but I doubt it will move that much over the final 375mm width - the slab is many years old, and well dried.

I still can't decide whether to tint the epoxy or not. One option I'm considering is to try the black tinted version to fill cracks on the underside (which will be forever out of sight, as the whole thing stands only 420mm high), then decide what to do on the top surface based on how that turns out.

If you load up the clear epoxy with sanding dust from the same slab, does the finished result (when dried) come out less shiny than epoxy alone?

Cheers

Hi Mr Brush

If you are using the untinted clear epoxy to fill the defects I am assuming you wish for the defects (gum veins etc) to become a feature of your slab ie you will be able to see, for example, the gum veins in their natural state through the epoxy and the final finish. If this is the case I would look at using a gloss hardwax finish, not a satin hardwax finish, as the satin finish may cause a blurry effect over the clear epoxied voids and gum veins and will look pretty poxy. (Poxy is not a real technical term but does convey my meaning pretty well). If the voids and gum veins are only small this will probably not be really noticeable but in your slab photos I did notice that there is a area of gum vein that is quite large. I think this would look quite good filled with clear epoxy but that is just my personal preference. I have a tendency to use timber that has natural defects and highlight them - although some people do find it detracts from the timber and looks almost 'poxy'.

I recently did a white cypress slab and filled the large voids with untinted luciclear resin and it came up a treat - until I finished it with Evolution satin hardwax oil. It took me a while to figure out what I had done ( resanded it several times) until the penny dropped. I then used a gloss hardwax as the finish and all the epoxied voids could be seen crystal clear. Just as an aside I found the Evolution gloss hardwax finish to be not very glossy when compared to say a poly gloss finish. I now use Evolution gloss hardwax for most of my pieces as I think it is pretty close to a satin poly finish.

If you haven't used luciclear before, make sure any defects that go through the board or intersect the edges and ends are properly masked up as the luciclear resin will find its way into cracks and holes you didn't know were there. Because of its long setting time it will just keep on running out the bottom of the void. I was amazed how much can run out of a hairline crack in a couple of hours.

You are on the money with the method to scrub, blow out and then seal the voids first with a thin layer of epoxy. This will prevent small air bubbles remaining trapped on the surface of the actual void/large gum veins when you do the second pour to fill the void.

With lighter coloured timber such a blackbutt I prefer the clear epoxy to fill voids and gum veins to highlight them rather than a big blob of black epoxy to hide them. However that is not to everyones taste. Your plan to use the underside of the slab as a test board is probably the way to go, try some with clear and then tinted epoxy side by side, put the finish on and see what looks best to you. Be careful you choose defects or voids that don't go right through from the underside to the top or your 'test' will be permanently on display on the top.

Sorry to ramble on a bit but I hope this helps

Regards
Twosheds

Thanks for the enormously helpful and detailed response

I'm reassured that at least one other person has tried clear epoxy on cracks in slabs, and the hints on using LuciClear are very helpful. I don't think any of the voids go right through the slab, but I was aware that this epoxy is VERY low viscosity. I was actually holding off until we get a decent warm day to do the epoxy work, as it's a bit cool here in the southern highlands right now which won't help with the cure time. Are we talking hours or days here (at say 20C)??

I had been wanting to try the Evolution hardwax oil anyway, having only ever used satin Polyx before, so I'll get a small can of the gloss Evolution to give it a try on this project. Apply with a short nap roller??

Thanks again for your help with this !

Cheers

Hi again Mr B

The luciclear usually cures hard within 24 hours or maybe a bit longer if it's cold. But if you don't add quite enough of the hardener it can take 2-3 days or longer for it to harden enough to sand. It is pretty important that your mixture is accurately measured 2 parts resin to 1 part hardener. As you measure out the resin first then add the hardener, I found it was easy to mismeasure the hardener. Don't worry too much if this happens, it will still go off but it will take a lot longer to fully cure to a sandable hardness.

For large surfaces like table tops I use the small 100mm microfibre rollers with a short nap. They are basically throwaways. It costs more for thinners to clean them than to buy new ones. However I usually get a second coat out of them by squeezing out the finish from the roller, wrapping in glad wrap film to keep them airtight and prevent the finish from curing in the nap. Don't make the mistake of tipping the squeezed finish back into your tin, store it separately. It has a tendency to start to cure in the tin, along with the rest of the tin's contents, within a matter of weeks. It is a bit exxy to lose half a tin of finish this way.

For smaller areas I just wipe-on the finish with a lint free rag. I often use any of the squeezed out finish this way if it is relatively fresh.

As you can probably tell I've made one or twenty stuffups along the way, mostly because I didn't follow the product instructions properly.

Just realized I've started rambling on again.

Regards
Twosheds

These rollers are fantastic for hardwax oil. 4mm nap microfibre. I wrap the tray and roller (and any unused product) in a plastic bag and it's good to use for several days.

Thanks guys - I think those microfibre rollers are the same as I use for the satin PolyX finish I already have. Web page for the Evolution product seems to suggest sanding to 220-240 grit before application is sufficient, let the first coat dry and denib with 400 grit before applying subsequent coats?

I've ordered a 500ml can of the Evolution hardwax oil in gloss, sufficient for this project.

Cheers

With hardwax oils, the recommended sanding is usually to around 240g and no more. My understanding is that beyond 240g the hardwax oil isn't absorbed into the wood as readily and won't provide the same level of protection. My limited testing suggests that this is the case. Very hard timbers like spotted gum sanded to 240g behave differently than white oak sanded to 240g - the oak and more recently, QLD blackwood was much more water repellant after two coats. For hard timbers like our eucalypts, I'd stop sanding at 180g. I didn't remember earlier - but I do have a recollection of speaking to someone from Whittle Waxes about hardwax on our hard timbers and they suggested using the 2k booster as well.

When I initially did my spotted gum laundry countertop, I sanded to 240g and used the recommended extra thin Osmo product. It went on easily enough and seemed ok initially, but the water didn't bead up like it would on other timbers I'd used Osmo on. I resanded it to 180g, applied two coats of Osmo extra thin and then one coat of PolyX on top and that has worked well. I applied the final coat with a white scotchbrite pad, so the surface is very smooth to the touch. I think I'll have to do a yearly or 18 monthly maintenance coat. I haven't tried the maintenance spray they sell.

A brown tint to match the gum lines looks much better in blackbutt. If you want loud, modern and in your face, go black. For a more subtle finish and gentler on the eye, go brown.

Thanks again all - good job I checked about the sanding! My instinct is normally to go to at least 320 grit.

I think I've already decided to try the Luci Clear epoxy untinted for the cracks, to allow some of the fine detail (see original pics) to show through; I agree that black might be a bit over the top. I'll definitely do the underside first before committing to the top visible surface though !

Currently working on a design for the legs, which I'll mock up in some scrap timber before applying to my (limited) stocks of blackbutt.

Cheers