Build #1 - GR-1SF

[Rossco381]

I do a very light wet sand before last coat, or maybe second last depending on how happy I am with the lay of the clear. I find this gets rid of any small imperfections or contaminants.
My understanding is that the poly "melts" the surface of the previous layer and that is the adhesion process between the coats. Am happy to be told otherwise if i am wrong. Definatley would not be the first time. On the very first coat onto the stained timber i use a thorough but light dry sand at 240 and give it a really good blow down with the compressor.

[Markus F.]

I do a very light wet sand before last coat, or maybe second last depending on how happy I am with the lay of the clear. I find this gets rid of any small imperfections or contaminants.
My understanding is that the poly "melts" the surface of the previous layer and that is the adhesion process between the coats. Am happy to be told otherwise if i am wrong. Definatley would not be the first time. On the very first coat onto the stained timber i use a thorough but light dry sand at 240 and give it a really good blow down with the compressor.

I always assumed that poly only adheres mechanically to the previous layer and it thus requires light sanding between coats. I thought only nitro-lacquer adheres chemically by softening the previous layer. Might be wrong though.

Gesendet von meinem EML-L29 mit Tapatalk

[McCreed]

My understanding is that the poly "melts" the surface of the previous layer and that is the adhesion process between the coats.

Further to what Markus posted above, yes polyurethane is "mechanically bonded" vs nitrocellulose lacquer or acrylic lacquer which is "chemically bonded".
Whilst there is some degree of chemical bonding with polyurethane due the small amount of solvent content, it is primarily a mechanical adhesion and not the same as lacquers. The solvents use with lacquers are significantly "hotter" which will soften pre-existing layer and "melt" (chemically bond) them.

Mechanical bonding, as with polyurethane, dry sanding between coats is meant to abrade the surface giving the subsequent coat something to "bite into" as many say. The abrasion (creating scratches) is essentially expanding (on a microscopic level) the surface area for the new coat to make contact with (ie: adhere to).

Wet sanding has a significantly different effect on the surface than dry sanding. Wet sanding has a polishing effect and much less of a "roughing up" of the surface. By creating a smoother surface, it is decreasing the surface area available for the subsequent coats to make contact with an thus adhere to.

That said, there are exceptions to every rule, and sanding between every coat may not be required. However application method can be a factor in sanding frequency (spraying vs wiping/brushing; rattle can spaying vs gun spraying).

[Rossco381]

Ahhh. That's where I was wrong. I think maybe I just assumed chemically bonded when I read it bonded to the previous layer. Will see how it goes in a couple weeks time for its final finish. Definalely not gonna strip it all back at this stage.

[McCreed]

Ahhh. That's where I was wrong. I think maybe I just assumed chemically bonded when I read it bonded to the previous layer. Will see how it goes in a couple weeks time for its final finish. Definilely not gonna strip it all back at this stage.

You'll probably be ok, and I wouldn't strip it back at this time either. However if you ever develop any delamination issues down the track, this might explain why. Also, I'm not saying you will have any future problems, only if.

Hopefully everything works out, because it's a great looking guitar!

And FWIW, even with finishes that are reactive like lacquer, I still would not wet sand between coats. Dry sand if needed to fix something, but not as part of a repetition - coat, sand, coat, sand, coat, sand etc.

[Rossco381]

With the rain and generally crappy day in Brisbane I could no longer resist. Had to have a look and see how it might come up.....
Think I am gonna be reasonably happy with the look of it!!

[Trevor Davies]

That looks great. Well done.

[McCreed]

With the rain and generally crappy day in Brisbane I could no longer resist.

Yes, I concur! And tomorrow's supposed to even wetter! Up to 100ml.

Don't know if you saw it, but I posted in another thread about applying finish on a neck. Or at least my plan was finish it this weekend! To dang humid! Have to wait 'til later in the week (hopefully).

Edit:

BOM now say up to 120ml tomorrow!!!

[Rossco381]

Pretty ordinary easter weather! Actullay is normal easter weather come to think of it.120ml??? Can't paint, can't mow. Just gonna have to sit back with a couple beers and the acoustic to practice I am thinking. Bugger!

[McCreed]

Pretty ordinary easter weather! Actullay is normal easter weather come to think of it.120ml??? Can't paint, can't mow. Just gonna have to sit back with a couple beers and the acoustic to practice I am thinking. Bugger!

We do what we gotta do!