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I want to share and get some feedback on something I tried this afternoon. I've been pore filling using a UV cured gel. I find pore filling exasperating. I know you're supposed to be able to get it done in 2 or 3 coats but it always takes me closer to 10 before all the pores are filled. And this is with any filler I've tried including Z-poxy. Today was the same story. I'd put on a layer of the UV cure gel, spread with a spatula, cure it and see that the pores were still not filled so I'd scuff with 220 and repeat. Finally I got the job done, but as I said, after about 10 coats. This ends up taking a lot of time and effort, primarily due to the scuffing part of the procedure.

Then I recalled something I read a long time ago. I think it was on a blog about the UV cure stuff from Admiralty Coatings where someone was asking about how well the stuff hung on vertical surfaces. Someone suggested applying a thin coat, doing a partial cure, then applying some more as a way to get a thicker hanging coat. So I thought I give this a try on some scrap wood.

I applied my usual pore filler coat spread as thin as possible with a spatula. Then I put the UV light on it for less than a second. I confirmed that it was not fully cured by putting a finger on it and seeing that it was still greasy and left a finger print. Then I put on a blob of the gel for a second coat. I wasn't sure what was going to happen. I thought perhaps that I would end up just pushing the stuff around with the spatula without any actual build up. But just the opposite happened. I noticed that as I spread it around, a lot was getting used - similar to when you put the first coat on bare wood. So it seemed like the not fully cured coat was grabbing a lot of what I was spreading next. After I spread it out, again as thin as I could, I did a similar partial cure with the light. I repeated this for a total of five coats at which point I did a full cure. Much to my surprise what I had then was not just all the pores filled, but a very thick covering. I sanded it back to level and marveled at how fast it had built up. I did it again on another piece of wood with the same results. It was way thicker than I would need for just pore filling and my next step would be to try three coats to see if that would be enough. Perhaps even two coats would do it.

If this works like it seems so far, pore filling will now go very fast and easy. However, I'm no expert on how finishes work and I don't know if this partial curing procedure is a viable approach. Can anyone tell me if this is a legitimate thing to do or am I violating some principle of working with UV cured polyester?

Thanks,
Pat

_________________
There are three kinds of people:

Those that make things happen,
those that watch things happen,
and those that wondered what happened.

Well, I'm not an expert, but...

My only concern would be that the thickness of the coat would possibly prevent the undercoats curing entirely, potentially creating delaminating issues.

Should be easy enough to test though by snapping your test pieces and checking the edges, or bending, twisting etc, cutting an x through to wood and trying to peel it up with tape. I'm sure you know the drill.

Fwiw, at Larrivee we used blue kitchen sponges to spread it, not a squeegee. The coat was very rarely dead flat before spraying.

Pat, sounds interesting. I've never tried UV cured anything, but a do kind of a 'partial cure' on epoxy. I read at one of the manufactures sites that epoxy bonds better between coats if you apply a second coat before the first is completely cured. I usually apply two coats in a single day, (sometimes three if I start early enough). I apply the second coat when my hand doesn't leave any serious indentations. Seems to work fine.

The UV cured product you use might be the same. Ed's idea to test sounds like a good one.

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Joe Beaver
Maker of Sawdust

Thanks for the responses. I will definitely be doing some adherence testing. I've tried many ways to apply the pore filler: squeegees, credit cards, rags (which is the recommended way by the distributor), razor blades but, I must admit, never a sponge. Perhaps I will give that a try. What seems best so far it the glue spreader that came with the glue I got for holding a jigsaw puzzle together. It seems to have the best combination of straight edge and flexibility to give a fairly uniform coat. It looks like just the top of a spatula that the glue company re-purposed for spreading their glue. I've also tried putting the filler on thicker as opposed to thin. That just leads to more problems trying to sand back level. I'm surprised to hear that at Larrivee the pore filler was rarely flat prior to finishing.

I'll keep on experimenting.

Cheers,
Pat

_________________
There are three kinds of people:

Those that make things happen,
those that watch things happen,
and those that wondered what happened.

Perhaps I should clarify. It would have been flat, as in no ridges etc., more often than not there would still be some evidence of pores which would easily be filled by the first cost of two of the high solids poly...

I'm just saying that I don't think the pore fill needs to be exactly perfect...

I have put the top coat on guitars without the pores filled completely knowing that what's left will be filled by the top coats. However my experience has been that even though they do get filled, and the top coat is made flat and buffed out etc, if the light hits the guitar the right way, I can see little pin-point sparkles from the pores that didn't fill. You have to look the right way to see them, and I've never had a complaint, but, never-the-less a perfectly done finish would not have these. Perhaps the effect is part of the finish I am using.

Pat

_________________
There are three kinds of people:

Those that make things happen,
those that watch things happen,
and those that wondered what happened.

Correct. If you're planning on two or more layers of epoxy, it's best to apply the subsequent coats before the first coat is dry. The reason is that the polymer chains are still forming during the curing process and the chains of the new coat will readily bind with those of the previous coat. It doesn't matter how many coats you do this way as epoxy cures due to chemical changes, not evaporation.

Once the epoxy is fully cured (polymer chains completely bound) the polymer chains of the new coat have nothing to which to bind, so you end up with a purely mechanical bond. If the previous coat was not sanded to give the new coat some tooth, then the new coat will simply peel away from the previous one.

I find this very interesting. I have almost always waited for a good cure before applying next coat of epoxy. And I too run into too many coats. IIRC, I one time did what u did, partial cures. And those came out much better.