Sanding Nitro Finish
 

I am finishing the GS Mini that had a crushed top now. I have sprayed the first coats of nitro finish on the top, and I have wetsanded the top with #400 wet with a drop of soap in the water. It feels very smooth to the touch, but I have some spots where the finish is still shiny. Does that mean I need to go over those areas again until I don't see any more shininess? I am worried about going through the finish so I would appreciate any advice before I proceed.
Thanks,
Clifford

If the finish is shiny, then the sandpaper didn't reach it meaning it is a low spot.

Depending how low, you may have to fill it to level (but you can't go too thick or it will cause problems).

My general plan of attack is to stop with the more coarse grits (400) when you have about 90% of the shiny spots gone. It lessens the odds of sanding through the finish.

As you continue to progress through the grits higher and higher to around 2,000 - you will continue to remove finish. Those shiny spots will disappear unless they are dings.

Then you can hand rub it with some sort of automotive rubbing compound. 2000 grit polishes out pretty quickly with rubbing compound.

Thanks for the advice. I am going to try higher grit sandpaper, and then do some fill if that doesn't work. I do plan to spray it 2 more times, and then cure it before buffing it out.

Unlike other finishes, lacquer will follow every undulation and imperfection on the surface when it dries. If there's only a few spots shiny, I'd fill them now, before re-sanding. If you can't get them out with 400, they won't come out with 2000. If you're also using lacquer to seal, I'd recommend using 320 free-cut paper until all low spots are gone (you may cut back to the wood), wipe down lightly with paper towel and naptha, then again with a dry paper towel,then reapply a couple coats of lacquer, and sand that to at least 600 before spraying final coats and curing.

If you are flat sanding and you have shiny spots and fear going through to the wood, then stop sanding, they are still the lowest point, apply 2-3 more coats of nitro at these locations and a full coat to the surface, allow to dry and repeat.

Steve

I will try the filling of the areas, and finish the second coat. I had not done any more sanding yet. Thanks.

Why do you say "Unlike other finishes"? What finishes are you referring to that will "level" themselves over imperfections?

you can get a very respectable finish using rattle can lacquer. But there are several things to keep in mind.
Because of the nature of spraycan lacquer, there is a lot more vehicle ( solvent) than there is solids in the spray - so even a full wet coat dries to a very thin coat. So in practical terms, you'll need to shoot 3-4 coats, wet sand ( I use 600 or 800 grit) then several more coats.
It also helps to let a few days go by between stages of coats. This gives the finish time to dry, and also do to shrink, or settle in so to speak. It may take 3-4 sets of spraying/sanding to get a good level finish.
Once I get to this point, I like to do a wet sand, then one final full wet spray coat. Then I will go back with 1000 grit, 1500 grit, and then 2500. I have recently picked up a 3000 grit sanding pad, that does a great job of buffing to a near satin finish. Meguiers Swirl Remover is a good fine polish that I like.
Be careful around the edges, it's easy to sand through to the wood there.
Don't forget to use a tack cloth between sandings, and watch for dust, gnats, etc! lol

May I ask, did you sand the wood, or scrape for the final smooth finish before spraying? Also, did you wet it, and resand or scrape to raise the grain?
Be patient, and enjoy the process. There's not many things more enjoyable than watching something take shape under your own hand.

Spraying nitro can be a bit forgiving. I don't do much sanding if any in the initial build coats. Once I am satisfied with the build layer, I then will level sand dry until all shiny low spots are gone. I then give it a final thinned wash coat, let cure for a couple weeks, then final level sanding if needed and on to buffing. If done right with care and patience, a terrific finish can be achieved even for beginners.

Slow drying mainly; the automotive 2-part urethane I use in the summer for one. I'm not talking about craters, just tiny pin-sized shiny spots. I may end up with a tiny undulation in the area when done, but it's shallow enough to be leveled. I don't start with anything coarser than 1000 when I sand the final coat.

Polyester coatings also "float out" almost dead flat, though likely not sprayed in a DIY setting, and letting it cure 24-48 hours in a totally dust-free environment is equally tough (I used to work at a shop adjacent a piano refinishing business, who did amazing work.) They only needed to sand to 240 grit max before spraying, and the polyester is "charged" leaving an even coat that floats out almost like glass.

Okay thanks for the info. That is why I consider Nitro generally forgiving, even if I get dust flakes, eyelashes or any other foreign matter, it is easy to sand out. I once found a hair completely encapsulated in the build coats with a magnifying glass, I chuckled and continued buffing.

IMHO, there is no reason to sand bare wood with sandpaper finer than 220.
You don't have anything to gain by sanding finer, and you may lose adhesion. The main difference between lacquer and other finishes is the solids content. Lacquer is mostly solvent, and as a result, it shrinks a lot when drying.

I sand to 320, then use shellac as a sealer, and then sand that to 400 before I spray my final coat. I spray one thin coat, let it sit about 10 minutes, then spray another coat, and let the finish cure. The finish is dust-proof after 15 minutes and cures either 4 or 8 hours depending on which finish/hardener I use. I can wet-sand and buff after 24.

As long as the surfaces are clean I have absolutely no problems with adhesion, even sanding the bare wood fine.

I agree, but for two exceptions.

I like to sand tops to 600. Doing so makes the "silk" in quartered woods really visually "pop". I've not had any adhesion problems on finely sanded tops.

Unrelated to finishing, I also sand wood surfaces that will not receive a finish - such as fingerboards and bridges - to 600. They take-on a different feel and appearance when sanded to that level. More recently, I use the Beall buffing system on unfinished bridges with the result that they look like they are lacquered, but are not, in a few minutes work.