Splotchy Sanding Coat

General Discussion
1

Hi guys, I’ve been a long time lurker on sways, and now I’m wrapping up my second project – both wood boards.

So far, I’ve learned everything I know by either “trial and error” or searching the archives here. I want to commend all the regular posters for creating such an incredible resource – I’ve probably logged more time in the sways archives than I have in the shop building boards.

But I’ve run into an issue with my sanding coat that I can’t seem to find in the archives.

Substrate is epoxy permeated wood (for adhesion), followed with standard PE glass job, 4 oz cloth, etc. Everything went fine until I got to the sanding coat. Basically, the coat has cured splotchy, with some areas hard and others VERY sticky, seemingly uncured. No way to even think about sanding this down.

I mixed the resin using a scale to measure catalyst and surfacing agent, so I’m pretty sure that the measurements were correct. I also thought I had mixed well prior to applying the resin. But I must have screwed something up. The only thing I can think of is that I put the surfacing agent in the resin, and then added the catalyst without stirring first. I saw a milky reaction occur when the two mixed, but I thought nothing of it and continued on.

My main question is how to recover from this point. I’ve got a surface that’s half cured and definitely unsandable. The only thing I can think of is using a solvent to remove the sticky portion, and then redo the sanding coat, but I can also see that going terribly wrong.

So, I’d love to hear any advice on how to save this project.

Thanks!

2

Just brush on another coat, this time mixing the surfacing agent first, THEN catalyst. This coat will make the first one polymerize. A bit more sanding, that’s all.

3

remember, you can apply epoxy to polyester but you cannot apply polyester to epoxy as there will

be no chemical bond.

3 options:

  1. “mechanical bond”, rough up the surface with 80 grit then apply polyester (fair results)

  2. “tie coat”, apply clear polyurethane onto epoxy (linear PU is best) then apply PE (good results)

  3. continue just with epoxy (excellent results)

4

Thanks balsa – worked like a charm the second time, and the uncured resin seems to have kicked, just as you said. That was a lot easier than I thought. Thanks for the tip!

5

Good on you. Glad I could be of any help.