Wet Sanding

[angry_jeremy]

I'm in the process of giving my guitar the final go-over, wet sanding, polishing, etc. First off the root of the problem is that I level sanded with p220 grit paper. It leveled nicely but I'm having a hard time getting rid of those scratches. I've gone back with 400, 600, 1000 wet/dry (used dry) then on to polishes but the 220 scratches remain. Then tried using a spray of water with the 400. Still 220 scratches. Both sanding rounds made little bits of finish build up on them which made marks as they were dragged accross while sanding. I stopped after maybe 10 seconds of sanding to scrape them off but they quickly came back. I then consulted Erlewine's finishing book. He suggests soaking the paper in water overnight to reduce the 'corns' on the paper. Did that, still loading up. The finish is maybe a month old, nitro lacquer. Any thoughts?

[mlennon]

If your paper is clogging up it normally means the finish is not yet fully cured but 220 grit paper is far too coarse to start for final wetsanding and polishing. Try to get rid of the remaining 220 grit scratches with 400 then 600 grit (adding soap to the water helps reduce clogging) and providing you haven't rubbed through your top coat, which is a real possibility, apply another few coats of clear. You will find that the clear will lay down much better and once fully cured do not use paper coarser than 1000 grit to level sand for polishing. I'd recommend starting with 1200 then 1500 and only use 1000 if you have very bad orange peel or runs etc.

[erikbojerik]

I would clean the whole thing off very thoroughly, and shoot another coat of clear over the 220 scratches. Maybe 2 coats. Let it cure, then level with 600 grit wet on a rubber block.

I think if you try to use the finer grits to remove the 220 scratches, you run the risk of sanding through...but if you still want to go that way, start with 320 and make sure all the 220 scratches are gone, THEN move to the next paper (repeat).

Don't just use water, add some Ivory hand soap. Use Ivory because it has almost no additives (lotion and the like), the soap acts as a surfactant which reduces clogging of the paper. You don't need much, just enough so that it foams up slightly when you wet sand.

[Maiden69]

Like Erick said, shoot more clear I would do more than 4 coats if it is nitro. And I would recommend wet sanding with 800 and higher grits. One thing I like to do is lay the clear thick, level sand with 320 dry, and shoot 2 more coats, then level with 400 or 600 and shoot one final coat wet sand with 1000 and start polishing.

[mlennon]

Will the scratches left by 220 grit paper cover/disapear when another coat of lacquer is applied? I have many years of painting experience and although I've never tried to cover anything less than 800 grit with clear it kinda goes against what I was taught as an apprentice automotive paintsprayer.

[Maiden69]

if it is nitro or any other paint that chemically bonds yes.

[erikbojerik]

220 scratches are pretty deep, so (especially if you're shooting from rattle cans) you may need more than 1 coat to fill them.

[ihocky2]

Another idea to reduce corning is to use something other than water to wetsand. Mineral oil works great, but causes sanding to take forever. I found that mixing a little mineral oil into mineral spirits gets rid of corning completely and the mineral oil prevents the mineral spirits from evaporating too quickly.

[angry_jeremy]

Yeah, good advice all. The only reason I tried 220 is that I saw Don something-or-other level sanding with 320 on the Stew Mac spray finishing basics video. I guess in this case an experiment that concluded to NOT use 220. The finish is quite thick, brushed on lacquer but I may re-coat to be on the safe side and to try to melt in some of the deeper scratches. I usually level with 600 wet and never thought of going rougher until I saw the video. Ah well, I'll know for next time. I'll try a little soap with the water before I recoat and see if that helps kill the clumps.

Thanks all.

[Prostheta]

Question for UK peeps: Does anyone have any recommendations for good "clean" equivalents to "ivory soap"?

[WezV]

Ecover washing up liquid works ok for me... just a tiny amount though

[Daniel Sorbera]

Also using a good sandpaper (I use micromesh) will dramatically reduce the corns building up on the sandpaper. I sand with just a little bit of water and don't get any build up at all, the only thing that comes off is dust that wipes off because of the water.

[Prostheta]

Awesome Wez - that's what we use anyway :-D

Awesome Daniel - that's what I use anyway :-D