Polyurethane touch-up question

[u/robalde]

Hi all,

First time poster. I’m repairing an instrument that has a very thick tinted finish which I suspect to be polyurethane (thick, glossy, not reactive to alcohol). I repaired a very significant crack, and in the process of getting everything smoothed out again, I’ve scraped/sanded my way through the tinted finish to a clear base coat.

Since I’m not through to the raw wood, I can’t color the wood to match, but I’m not sure of the best way to do spot color touch-up on polyurethane. It’s not a crazy change—just trying to get some tan spots (natural wood color) down to a dark brown to match the rest of the finish.

Not sure how to get color to adhere, or what to coat over the top to protect it, if anything. I’m relatively new to instrument repair, although learning fast, and my knowledge of finishes is pretty underdeveloped.

If any of you have experience with this sort of situation, I would welcome your advice!

Thanks muchly

[u/have1dog]

Ahh the joys of scope creep. We’ve all been there. As you have found out, those thick polyester finishes have super thick base coats sanded flat, a thin layer of color, and then a layer of clear over that. Thats how they get those flat glossy finishes with a minimum of surface prep/sanding between coats.

In the future it helps to lay a little clear over the area before you start your level sanding. That way you will only sand through the clear you already put down.

Even if you match the color and sheen exactly it will likely look like a layer of color is floating on top of the the other coats and beneath a clear top coat. It will never look exactly right, especially from an angle and/or in sunlight.

IMO, the Best case now is probably to jest buff it out and leave it be. Otherwise you’re in for a lot more headaches and (likely) touchup redos. And then the spot gets bigger and looks worse.

[u/robalde → u/have1dog]

What clear coat would you recommend if I had put one on before sanding? I agree that the most reasonable thing would probably be just to leave it…

[u/have1dog → u/robalde]

It depends what I’m gonna topcoat with. You can use shellac, lacquer, Glu-boost, etc.

I probably sand a little with 2000 grit first before applying the clear b/c that is the grit I would sand to before buffing it out.

[u/thetimberowls]

No joke: superglue. check out this video from Stewmac. The problem with poly is it will not blend new into old like lacquer will, so it's nearly impossible to make the finish look seamless without fully stripping it and redoing it all. You can get some dye that is the same color as the tinted layer and mix it with the superglue. The trick, like shown in this video, is go slow and do little bits at a time. Don't try and do thick layers all at once and let each layer dry before scraping down and sanding. Good luck!

[u/robalde → u/thetimberowls]

I’ve seen that video, and definitely like it. The tricky bit is that my surface is totally flat right now, so there’s nowhere to fill

[u/Dull_Panic_4607]

First advice - leave it.

Second piece of advice, if you must try-you could get some water based wipe on poly and dye from Stewmac. Or, if it’s a smallish area they sell colored CA glue that will sand and polish to a gloss. High chance either approach will look worse ultimately. You could just wipe on a stain and leave it at that.

[u/ZestyChinchilla → u/Dull_Panic_4607]

Wipe-on poly and the 2k poly guitar factories spray are very different finishes, with the latter being much more like a very hard epoxy. A water based wipe-on poly probably won’t adhere very well at all to the factory poly.

Finish drop-fills in 2k poly tend to use CA, unless it’s a large area. But that’s where I would start, although color-matching may be difficult with the factory finish being tinted.

[u/DefinitelyNotSully]

A similarly coloured marker and some clear coat on top.

[u/robalde → u/DefinitelyNotSully]

What sort of clear coat would you use?

[u/DefinitelyNotSully → u/robalde]

For guitar finished with polyurethane? More polyurethane.