Oil-based polyuerthane gloss over opaque black. Can I achieve this without having the scratch/scuff marks showing under the gloss?

[branded]

Hello,

Been struggling with this for weeks and have sanded down my guitar several times. First time I've done this, so be gentle.  It's my first (cheap) guitar, so it's sentimental to me and I simply want to just refinish its black gloss finish.

I have done the following so far:

* Primed and coloured with Rustoleum black gloss with about 6 coats of each. I haven't noticed any bad reactions with the poly and it's completely dried and cured for several days before applying the poly.

* Sanded/scuffed with 240 grit sandpaper as per the instructions on the can of Cabot oil-based polyurethane gloss.

* Brushed on the poly. I've also tried sanding down to the black coat again and tried many wipe-on coats of 50/50 poly/turps.

The problem is that I can see the sanding/scuffing marks under the poly when shining on it with a light. I might be pedantic, but I thought it should be possible to avoid this somehow.

There seems to be so many opinions out there. Some say I should have worked my way up to 1500 grit before the first coat of poly, but that would mean the poly not adhering.

I'm reluctant to try other finishes right now, because I've spent quite a bit of money already on this stuff and I want to use it. I don't mind the work, but I just want it to work!

Right now, I've sanded it back, applied the black base coast again and I'm trying a black pigment in the poly and I've done my first coat of that wipe-on. I'm hoping that the pigment will hide the scratches of the base coat. It's only wipe-on and I've only done one coat so far, so the scratches aren't filled it yet, so we'll see how I go with more coats.

In the meantime, can anyone here demystify this for me? Is there absolutely no way I can achieve a brushed/wipe-on poly coat without the sanding marks showing under it? I know a lot of luthiers use the same poly, but I've only seen it ever done with guitars that have the wood grain showing and not opaque colours and thought that maybe wood grain hides the scratches?

Anyway, I really appreciate any help, thanks!!!

[henrim]

Clear coat doesn’t hide scratches, if anything it amplifies them.

You need to have a level and scratch free surface before you apply clear. For a solid glossy black finish I would rather use hi-gloss black paint and no clear.

Anyway glossy black is probably the hardest color to get right. Every tiny spec of dust will be visible in the surface.

[Bizman62]

3 hours ago, branded said:

The problem is that I can see the sanding/scuffing marks under the poly when shining on it with a light. I might be pedantic, but I thought it should be possible to avoid this somehow.

There seems to be so many opinions out there. Some say I should have worked my way up to 1500 grit before the first coat of poly, but that would mean the poly not adhering.

As you said you sanded up to 240 grit. In my opinion that's a bit on the coarse side, yet it shouldn't leave single scratches. My guess is that you've used a heavy hand and haven't cleaned the sandpaper too often. Let me explain: When pushing hard you're increasing friction which heats the surface which then will melt and reharden on the paper. So instead of a uniform layer of 240 grit abrasive you have lumps of rehardened mix of abrasive particles and finish. Adequate pressure for sanding is the weight of the block and your fingertips. It's more about repetitive wiping than forceful scraping. Same with machines, the weight of the sander (random orbital or vibrating) is plenty enough, you're just moving it around to prevent sanding through in one spot. No extra pressure. And clean off any buildup from the paper every so often.

Another trick for minor scratches is to sand along the grain. That can masquerade some scratches as parts of the natural figuration.

240 grit between layers of "paint" of any sort seems quite rough to me. For bare wood that can be enough depending on the finish but for plastic (yes, all poly is liquid plastic) I'd go a bit higher. A satin finish is the goal, the next layer should adhere to it. So instead of 240 I'd go up to 400 or even 600, potentially wet sanding. 1500 sounds high until maybe between the last two layers of clear or rather before buffing the final coat. In the latter case I'd go up to at least 2000, depending on the buffing method. For hand buffing I'd go even higher before using polishing compounds with mild abrasive.

Oh, and welcome!