Sanding between coats of lacquer

[Richardwoodhead]

I'm in the process of finishing over 20 chairs (jarrah). I've been working on these for about 2 months non stop and it's the home stretch (and I'm a bit knackered). I'm finishing with a 2 pac acid cat lacquer (Becker Acroma DM 307). In previous discussions re lacquer finishing some have suggested you can apply multiple coats without fine sanding between coats (I think Durwood was one person who did this.) The theory is that the thinners "melts" the new coat into the previous coat.

I followed this idea up with Becker Acroma and other local finishers, who felt it was not the way to go (i.e. no sanding) as you would not achieve a good "build" and smooth finish. But after hours (and hours and hours) of detail sanding - I'm beginning to wish it WERE possible not to sand back.

I tried multiple coats on one chair without sanding. But it didn't look or feel as nice as the sanded back result. Maybe the no sanding technique requires some final rub-down to achieve a good finish.?

Any ideas / experience on this would be appreciated.

Thanks,

Richard

[journeyman Mick]

Richard,
while the succesive layers of lacquer will melt into previous layers it's still neccesary to lightly sand between coats to remove any nibs (small rough particles). With acid cure there's a recoat "window" during which new coats will chemically etch into previous coats. After this period you need to mechanically roughen (heavier sanding) in order to key succesive coats in. If you stay within the recoat period only light sanding - enough to remove any nibs, is required.

Mick

[RETIRED]

I find a rub down between coats with a scouring pad is sufficient.

http://www.woodworkforums.ubeaut.com...read.php?t=485

[echnidna]

Just a quick light rub to de-nib then recoat

[Richardwoodhead]

Thanks guys. I've been carefully re-sanding with 320. Takes ages. I'll definately look into the scouring pad technique. I assume with a scouring pad it's not too important to go with the grain? Just rub all surfaces quickly to de-nib? Mick & Bob, what do you rub back with? , your post via the link was excellent. Thanks again.

Richard

[echnidna]

wet and dry between 250 and 320.
Just a quick light circular scrub

[journeyman Mick]

Usually 320, just a quick rub, all you want to do is remove the little nibs. I have used scouring pads as I got a huge supply at one time when doing a boat fit out, all the left overs. Have run out now and just gone back to the 320.

Mick

[Shane Watson]

Hmm...if your using acid cat lac, then no it will not melt into the previous layer. However with NC lac, then yes the thinners does help melt into the previous layer. BIG difference in products.


You do need to sand in between coats so that the coats 'key' to each other and also (and probably more importantly) you get a much better finish...Preperation is the key to any finishing job and the prep work dosn't finish after sanding the timber ready to apply the finish.


I sand inbetween coats with 240 or 400 when using acid cat, but if using poly-u then I sand to much finer grits.

[Grant_33]

Yes there is a big difference between true nitrocellulose lacquers and acid catalysed finishes both single pack (precats) or two packs.

As acid cats cure by a chemical reaction that commences after the majority of the solvents have evaporated from the film (esp the alcohols) they can be recoated without sanding for a short time but if left for a longer time then sanding is essential. This "window" will vary from product to product. Your paint supplier should be able to provide this information.

If you are within the recoat time then sanding is required only to remove nibs or dust or other imperfections.

True Nitrocellulose lacquers (which arent that common these days) dry only by solvent evaporation and can be recoated at any time without sanding. This makes later repair or refinishing easier. They are not as tough, hard or hot liquid / alcohol resistant as acid cats but imho give a much warmer appearance to the timber. Another big bonus is they dont release formaldehyde on curing (a nasty little feature of any acid cat lacquers that use melamine or urea formaldehyde resins - ie most of them).

As to sanding some people also seem to like steel wool although this can cause black spots on certain timbers.

Regards
Grant

[Richardwoodhead]

Thanks Mick, Shane, Grant. This info is what I've been chasing. Becker Acroma from Sweden prize themselves on producing the lowest toxicity lacquers (apparently - though I can't really comment). They have been telling me I must sand between coats to get a correct result - but no-one has explained why. Now I know.

Although just by trial & error - sanding back produces a hugely better result. Wish it didn't involve so much effort on fiddly pieces. (Although the actual spraying, after all the sanding is done, goes incredibly fast. And "hey presto" - beautiful end product. You just gotta love a good lacquer finish when it's done right. Smooth, even, silky soft feel.)

I used a scouring pad (rather than sandpaper) on a workpiece this morning, but noticed there was no "cut back" of the timer, no fine white powder etc. Following Shane & Grant's input, I assume the scouring pad technique is limited to non acid curing lacquers? As the pad really doesn't generate the ability for subsequent coats to "key in"?

Along the lines of Grant's comments... Becker Acroma recommend a "double header" spraying technique. After completing one coat, the workpiece is then immediately sprayed again. Within a few minutes of the initial spray. The solvent isn't really dry and the second coats flows right into the first.

Really appreciate this feedback.

Thanks,

Richard

[soundman]

"scourers" are used extensively in many areas of finishing.
They don't appear to be as agressive as paper but it is a mistake to think that they arent cutting because you don't see as much dust.
Most of the dust will end up in the pad. They also tend to sort of burnish than sand. They are great because the conform to the job well and dont tend to cut thru on edges as easily as paper.
Remember if you go to a refinish supplier you can buy about 4 grades of "scourer" and the sheets are much bigger ( like 3 times the size) and cheaper than buying at the supermarket. The material is also more consistent.
I recon the proper scourers are great, but they are very different in how they work & what they do than sand paper.
cheers.

[Richardwoodhead]

Thanks Soundman. I noticed the white powder in the scouring pad and was wondering about that.

Will search out scouring pad options at the local paint shop. Don't have a "refinish supplier" as such. Do you get your different grade scourers from a paint shop? The supermarket ones were fairly pricey.

Richard

[KevM]

I use the maroon Scotchbrite pads. Will check out details from box tonight. Purchased from Blackwoods by the box http://www.blackwoods.com.au/content...20&do_search=1 or from Timbecon by the each http://www.timbecon.com.au/products/...ing-490_0.aspx .

Kev M