Help with finish issue

[thistle3585]

I spent a couple days wet sanding an emando and went to buff it today and there were finish fractures throughout the top and sides. I'm assuming its from moisture getting under the finish from holes for the pots and pickups since they are radiating from those areas. Up until this batch of instruments, I had been using Seagraves and never had this issue but recently switched to Mohawk and I've had it happen on two of three instruments that I've finished with Mohawk. I have been wet sanding progressing through 500, 1000, 1500 then 2000 with a mixture of water and detergent. I'd like to stick with the Mohawk. I like how it lays down over the Seagraves. So, I'm interested to know what wet sanding process others are using for a sprayed lacquer finish. Is anyone successfully using a dry sanding process?

[rowka]

I’ve had this happen with nitro on electric guitars. My solution was to butter the crap out of the holes with Johnson’s paste wax before I started wet sanding.

Well, maybe fractures isn’t really what I’ve had, but the wood would swell a bit from the water along the grain (from absorbing into end grain exposed in the holes). I’d wet sand to flat, but after the wood dried and returnd to normal dimension, there would be divots.

[Greg Mirken]

I'm not convinced your checking problem is from moisture. However, I have always used mineral spirits to "wet" sand [learned working at Dobro in the 70s]. Works great; no moisture issues.

[HoGo]

I'm not convinced your checking problem is from moisture. However, I have always used mineral spirits to "wet" sand [learned working at Dobro in the 70s]. Works great; no moisture issues.

You may have a thicker finish deposits at the edges of the cavities causing problems. Swelling together with thick lacquer may result in checking.

[thistle3585]

Thanks. Here is the problem. I am assuming that thicker deposits at the edges should be resolved by more thorough sanding between coats in that area to reduce buildup?

[rcc56]

It looks to me as if you might be applying your lacquer coats too heavily, or not thinning it enough, or recoating too quickly.

When in doubt, extend your drying time between coats and before rubbing out.

You mentioned that you have changed lacquer brands. The Mohawk may require different preparation or a different spraying schedule from the Seagrave. For the record, Jim Grainger preferred Mohawk, and he had quite a busy shop.

I also prefer mineral spirits or naphtha as a sanding lubricant for final polishing. For scuff sanding between coats, I use 320 dry.

[thistle3585]

It looks to me as if you might be applying your lacquer coats too heavily, or not thinning it enough, or recoating too quickly.

When in doubt, extend your drying time between coats and before rubbing out.

That makes sense because I had reduced it with retarder, because of the amount of rain we've been having, but was still spraying on the same schedule as if I hadn't added retarder. So, recoat time probably should have been extended. I didn't have this problem on my very first instrument where I hadn't used the retarder and I only started using it after having applied two coats on this instrument because I was getting some blush. Thanks for the insight. Will look at my spray schedule going forward and see if that resolves the issue.