I have been using UV cure poly laminating resin for a long time and I always used surfacing agent in the sanding coat. Often my sanding coat still remains tacky and difficult to sand. It gums up the sand paper quickly. Recently I tried just buying UV cure sanding resin, I am assuming it just has the surfacing agent in it already. However I am still having the same problem… a tacky finish.
I try to make sure that the resin is warm, sometimes I put the can in a bucket of hot water to make sure the surfacing agent is not cloudy but it doesn’t seem to help.
What am I doing wrong, or are there any tips I am unaware of?
From my (very limited) experience, after you brush on a UV hotcoat, you should let it sit for a few minutes to allow the wax to rise to the surface, before exposing it to sunlight. The waxy surface is what allows the resin to cure hard enough to sand.
Ryan is right, but with one extra. After you have brushed/layed off you hotcoat, carefully pick your board up from underneath, walk it out into direct UV-sunlight. Holding the board in direct UV, count to ten or twenty. Now walk the board back inside and set it on a rack with indirect light. By indirect I mean just whatever artificial light may be in the room. The hotcoat should start to look slightly milky. Sort of an oil on water look. This is the wax in the resin rising to the surface. After 20 minutes or so take the board back out in direct sunlight and leave until the hotcoat is rock hard… Do the other side exactly the same way. Proceed to sanding. You have not been letting your surfacing agent(ie wax) rise to the surface. If it’s tacky when you sand it; it is because you are sanding laminating resin. The wax is below the surface. When you were using MEK, you were probably not using enough. You can tell if it is hard enough by pressing your finger nail into it. If your nail won’t cut it, it’s hard enough to sand. With MEK you have to wait longer before you can sand. With UV if you do it the way I described you can sand right away. With practice you’ll be able to grab the board with your hand and tell that it’s ready to sand. PS. The few seconds in direct light sets off the process that allows the wax to rise. You put it back inside to give it time to rise. I said 20 minutes, but it could take a little longer. It should look kind of milky all over. You do not want to exspose it more than a few seconds to direct light. If you let it get hard withou allowing the wax rise, it will never sand. I hope you are working in a room that is heated to about 70 F. That is the most common mistake made by many on this site. They wouldn’t print info on working temps in product descriptions if they weren’t relevant.
Thanks jrandy. I did an MEK hotcoat yeasterday(non UV). I had the room up at 70. With the Cadet heater it doesn’t get much warmer than that. Because of all these discussions I was watching it just for perspective. I could see it slowly changing. It went from glossy to satin. In a short time it evened up to a flat/satin. I started sanding it last night. It sanded and powdered up nicely. Used the same piece of paper on my Milwaukie to do the whole bottom for first pass. What I started with was a quart of Poly laminating resin 249AOB . Not prepackaged sanding resin. I added a full cap of Surface Agent(ie Styrene Wax Solution). The cap was a standard size cap off of a pint(16 oz) can. I shot 15cc Plus of MEK into the Quart of Silmar. Sanded nice.
The most common reason for a gummy hotcoat that won’t sand easily is over brushing. This happens with either type of resin, UV or standard catalyst.
Once the wax rises to the top disturbing it by brushing more will eventually prevent that layer from forming. Poly resin is air inhibited. You need a barrier between it and the air for it to harden completely. That’s what the SA does. It floats to the surface and the wax creates a barrier.
Shake resin can well if it is pre-mix. If mixing yourself make sure your ratio is correct and that you mix it well. Flood it on, brush it out, then cross brush side to side then end to end. Work quickly. Walk away for 5 minutes while it flows out and the wax rises. Take it into the sun briefly then back into the shade for a couple moments. Then back in the full sun. Follow this method and you will never have a problem with UV Hot Coats.
Thanks again for further confirmation, from Matt, Mako, and Sammy, of advice I am getting for a successful hot coat. This all seems like really sound advice, and it is certainly different that the way I have done this in the past. So I am looking forward to trying out a “test patch” before I glass another board in about a week or so.
When I mix in my wax additive I err on the side of having a little more than the can says. Also if you’re mixing your own like I do make sure that the wax hasn’t coagulated in the container. I usually warm the container up in hot water and then shake it up real good before adding it to the resin.