How much to sand back?

[Fitzroy]

I’ve finished a desk top in Polyvine varnish. I’ve put three coats on and lightly sanded back with 240 grit between coats.

I sanded the last coat very gently with 240 then 400 grit. It feels smooth but it’s still ridged under finger tips, which I guess is the brush lines.

I’d like a ‘glass smooth’ish finish, is this possible with an acrylic varnish, do I just keep gently sanding until it’s finger tip flat then go up through the grits to smooth?

Don’t want to ruin my hard work by being over exuberant and trying to achieve something that I can’t.

Cheers

Fitz.

 

[profchris]

Essentially yes, if your finish layer is thick enough that you won't sand through. But if your first coat of varnish raised the grain, you might hit wood before you get to glass smooth.

I dont know whether that particular varnish will polish up to a mirror shine, but a decent gleam should be achievable.

One thing you might try now is to buff up a section with T Cut or some other polishing compound. I use this on my ukuleles when I'm not trying for a mirror finish. This could give you a nice soft shine if you've sanded enough. If you dont like the effect, 30 seconds fine sanding will remove it.

 

[Fitzroy]

Thanks for that. I raised grain and sanded back before the first coat. I guess if I go too far I’ll just have to add more coats.

F.

 

[ED65]

I’d like a ‘glass smooth’ish finish, is this possible with an acrylic varnish...

If it cures hard enough absolutely. Everything that goes hard enough can be polished, everything.

But how long ago did the Polyvine go on? If it's only a day or two since the last coat it's much too soon to be thinking about doing this. With drying times in the range of hours you want to wait days at least before you try to polish up as this is how long a proper cure takes. Some users wait a fortnight, same way they'll wait a month+ if using oil-based varnishes.

Also I doubt three coats is enough for this, but it depends on the product and how thickly you applied it naturally.

...do I just keep gently sanding until it’s finger tip flat then go up through the grits to smooth?

Essentially yes, but I think you're making a big mistake starting with 240. You want to use something north of 400 for the flattening stage generally (well north in the opinion of some), although I understand you do have quite a bit of texture here.

And just in case you don't know it matters greatly if you're P-rated papers or not.

 

[Fitzroy]

Thanks ED65. I’d noticed in knocking back the first coats that I didn’t get only a fine dust, I got dust but also some of the dust agglomerates, so I was suspicious the coat couldn’t have fully hardened.

I’d seen a few videos where people are starting with 600+, so picked up there was some contention in this area.

Paper is all P graded wet and dry I think as it all says P600 etc. I’d noticed previously that was my wet and dry was way more uniform than other abrasives but not realised that the a P signifies a standard (just read about FEPA), which as a geeky engineer I love standards, so thanks for that!

As of 10mins ago it now has four coats and I’m out of varnish. Will leave it a few days than flatten as much as I dare. My boy is clamouring for his desk back so can’t leave it in the worship too long!

Fitz.

 

[profchris]

If the last coat has just gone on then just give it back to your boy once it's hard enough not to mark. A few days is nowhere near enough.

Kidnap it back in two weeks minimum, four would be better. Make it a father and son project, with him doing all the grunt work!

Seriously, the finish must cure first, otherwise sanding won't get you anywhere near what you want.

 

[ED65]

I’d seen a few videos where people are starting with 600+, so picked up there was some contention in this area.

Paper is all P graded wet and dry I think as it all says P600 etc. I’d noticed previously that was my wet and dry was way more uniform than other abrasives but not realised that the a P signifies a standard (just read about FEPA), which as a geeky engineer I love standards, so thanks for that!

So glad I mentioned it! You'll have likely read this already but 600 is a great example of what I was getting at as it's equivalent to nearly P1200 For the coarser grits the numbers roughly line up and between about 60 and 220 can be considered equivalent (close enough for government work anyway) which is handy for us as this is the range most used in woodworking.

Older references stress that the P-rated paper should give a more consistent scratch pattern because of the tighter grading, I don't know if this is always true any more with modern production methods and the very high quality of some CAMI papers/screens etc.