spraying shellac and lacquer

[Prashun Patel]

As far as I know, these finishes do not cure; they just dry. Coats melt into each other to create a single coat.

This got me thinking: if you are spraying, then is it ok to spray the next coat as soon as the first starts to set and is still tacky? I usually wait until the coat is fairly dry to the touch and sand able before putting ton the next coat. But is this necessary? As long as it doesn't run, what's the harm in spraying coats within a few minutes of each other?

[Art Mann]

I put on successive coats of lacquer in about 30 minutes - sometimes less. That is all the time it takes for the material to dry to the touch unless it is a very heavy coat. I don't like to spray wet on wet because it seems to increase the amount of dust nibs and roughen (Is that a word?)the finish. I usually put on 4 coats and I don't do any sanding until just before the last one. I usually go over the piece lightly with 400 grit to get rid of the nibs. The last coat makes for a silky smooth finish. I need to add that I am typically spraying small objects like boxes, plaques and indoor signs. Big furniture pieces may work differently.

[Brett Robson]

I know nothing about spraying lacquer, but in my shellac spraying experience, if I don't give each layer at least a hour or two to dry, the finish stays soft for quite a while. I'm assuming the film gets too thick and the alcohol can't evaproate very well.

The exception to this is for the first couple layers, which are being absorbed into the wood. For the first two or so, I usually do spray successive coats within a few minutes. After the wood gets starts to get sealed, I give it a little time between coats.

[Prashun Patel]

Brett, spraying 4-5 coats 1.5#, is still way thinner than a single brushed coat of 3# shellac. I do respect that after the last coat goes on, it should be given ample time to dry, but for me, it's natural to let it dry over night. I just did this on a set of cabinets, and I don't find I'm having any trouble sanding it to a white, fine powder as it should.

Correct me if I'm wrong: just because the wood absorbs the first coats, it doesn't mean the alcohol has evaporated. It just means the mix penetrates into the surface enough to minimize the stickiness we feel. If it's ok to recoat quickly here, then it supports my theory that the only reason we wait is for ease of application of the next coat. When spraying shellac, it dries within minutes enough to resist pooling and sagging upon the next coat.

Now, something I DO notice is that you can't force the drying by heat. I tried that and found that heat causes blistering. So, it's the final drying that requires time - not the initial ones.

[Brett Robson]

Correct me if I'm wrong: just because the wood absorbs the first coats, it doesn't mean the alcohol has evaporated. It just means the mix penetrates into the surface enough to minimize the stickiness we feel. If it's ok to recoat quickly here, then it supports my theory that the only reason we wait is for ease of application of the next coat. When spraying shellac, it dries within minutes enough to resist pooling and sagging upon the next coat.

I'm certainly no authority on spraying shellac, but at least in my experience, after the wood's been sealed, if I spray too many layers too close together, the finish remains soft for many hours. It's not sticky or tacky, it just has a soft feel and isn't hard enough to sand.

I'm not sure that's really a problem though, if you're planning on letting it sit for a day or so before any final sanding or rubbing out of the finish.

[Howard Acheson]

>>>>> these finishes do not cure; they just dry

Yes, shellac and lacquer dry by the evaporation of their thinners. Wen the thinner evaporates completely, the finish is dry. No "curing" takes place.

As to your question, many professional finishers will spray the next coat directly on top of the prior coat. Howeve, this works best with finishes that are designed for or intended to be sprayed. Some finishes can be problematic if too thick a film is built up. Personally, I let the underneath coats fully dry. They dry fast so little is gained by immediate re-coating.

[glenn bradley]

First the direct answer; I apply a next coat in about 20 - 30 minutes here in the desert valley.

As Howie points out, shellac too thick and you get a slow carrier evaporation. The outer surface gasses off quickly and creates a 'hard candy shell' that slows the evaporation of the carrier in the under-layers (is that an industry term? :)). In my limited experience the shellac has to be pretty thick to "never" fully harden but, I do have large runs on squeeze bottles that have remained soft for many months despite my dry climate.

As mentioned, when I spray shellac it is very thin when compared to a padded coat. I go through a lot more product to get the result I am after when spraying. Many more coats are required and I find this fatiguing both mentally and time-wise so I pretty much avoid spraying unless it is absolutely necessary. Spraying is a good method for hard to get to areas as long as there is a through-opening . . . that is, not a cubbie in a desk or a corner in a shelf unit but, like spindles on a chair :).

[Prashun Patel]

Thanks Guys. Glen, I understand about the candy shell effect. (in fact, I think some pills and candies actually use shellac!!!) Therefore, I wouldn't dream of BRUSHING on two coats of anything too quickly. But as you mentioned, a few spray coats is equivalent to only 1 brushed or padded coat, so what's the harm in spraying them in quick succession? My experience on this project has been that what I'm suggesting is fine: spray a few coats on within minutes of each other, then give it over night to dry.

My project is sufficiently large such that by the time I get to the last piece, the first has started to set. So 'rushing' allows me to do a single session for longer. This is more efficient for me.

I do notice one thing: the second coat is more prone to orange peel than the first. But when all the coats are thin, there's no issue.