Buffing polyurethane finish

[boneaus]

I have applied 5 coats of Cabothane water based satin polyurethane to a marine ply speaker box project by HVLP gun. Even though I sanded between coats with 240-400 grit, the final finish is not as smooth as the sanded raw timber. The grain is somewhat raised and I would like a smoother finish. How can I buff the finish smoother?

[Drillit]

Hello Boneaus,
I think water based poly is not a good finish. Personally I would have dampened the ply to lift the nap and then final sanded with say 240/320. Ply is very absorbent. I would have then used a sealer (proofseal from Feast and Watson) let it dry and then applied the top coats with light sanding between coats. I think the problem is that the water based poly continues to raise the grain. I would therefore, give a heavy sand, give it a coat of proofseal and let it dry and then recoat with your poly (say 3 coats) and lightly sand betwen coats and make sure that you wipe if clean between coats. Hope this helps. Drillit. PS - You can always pre-test on a piece of scrap.

[soundman]

now just to check some facts.

Are you sure it is the timber fibres that are showing thru or is it dust and other crap.

I think you are sanding far too fine, with most film forming finishes you are wasting your time sanding much past 180 or 240 even between coats.

Polyeurathane is pretty thick gluggy stuff and all you do by sanding too fine is compromise adhesion.

persoannly after 5 coats and sanding between I doubt that the roughness you have is timber , its probably just dust and stuff...have a close look.

once cured well polly rubs out pretty well with wett and dry.

I have never been able to get a realy good smooth finish out of polly straight off the gun or brush...the stuff dries soooo slowly that every bit of dust settles on it..and you are working in water bassed....the insects love the stuff and will come from miles to comit hari kari in water bassed paint.

Have a go wet sanding with 600, 800, 1200, then go a good stiff rub with the U beaut traditional wax and an old polo shirt, buffed out with the finest fat ladies undies you can obtain.

be carefull with the water..don't slop it arround and keep wiping up with a rag.....just wet enough to do the job.

If you can not obtain fat ladies undies ( the best polishing rag known to man) a worn cotton t shirt will have to do.

give it a go and report back........show us some pictures.

cheers

[nrb]

Are you using your Ploy out of the container by dipping your brush or rag(with wipe on) if so all you are doing is transferring any junk back into the clean can,it will get dirtier and dirtier the more you use it .
If you start off by pouring of the amount you need into a clean container then using it out of that.
At the end of that coat throw away whats left,if you return it to the original can all you do is make it dirty,you will soon become good at estimating how much to decant so not to waste.
On larger surfaces use a foam roller and apply a good wet coat,some areas will start to dry,don't be templed to go over again,once you have applied that wet coat leave it alone to dry. Throw away the foam roller,they don't cost much in fact you will most likely spend more on turps to clean it and you never really get it clean anyway
I forgot to say use a tack cloth before you start,I often use a clean cloth dampened with turps,you will see the junk that comes off,allow to dry then go to it.
Gentle use of a heat gun can dry off the turps if you are in a hurry.
Hope this will help you get a better finish.
Sand with 240 between coats

[Humphrey9999]

I agree with soundman. 1200 then wax. That's how I get the highest gloss out of WOP.