HVLP gun questions

I understand what you in the US call lacquer is what we in the UK call cellulose. I have sprayed a number of cars in cellulose in solid colours and metallics having taken advice from a semi-professional bodywork/spray friend and while sanding each coat might be done on primer coats, using contrast coats to highlight high/low areas, would not be done on top coats unless a show car finish was required. All the hard work being in the primer filler preparation finally ending up at P600 IIRC (a bit finer than US 600 AFAIK), the topcoat application being the easy bit. Multiple top coats being applied rapidly one after the other as required and the paint then allowed to dry. This does assume that you can spray the paint straight from the gun and get a high gloss finish without runs which I have been able to do. Many shy away from applying the paint that thick and suffer orange peal effect which subsequently needs to be flatted and polished.

Regarding shrinkage I wonder how many coats that would take. I have applied 10 coats of cellulose topcoat before and never had issues with that cracking. The only time I have seen paint coats crack is when preparation has been poor or incompatible paint system have been used.

Yes -- based on my understanding from an ill-spent youth reading British car books.

Right. But if you're good enough to get a good gloss from lacquer sprayed one coat on top of the other, without sanding, you probably could spray acrylic enamel and get a tougher finish in one coat, to begin with.

I think the cracking is a result of spraying thick coats on top of each other without allowing complete drying. My slow, thin-coat approach never produced a crack that I know of.

It is possible that you're referring to nitrocellulose lacquer, which I don't think has been used in the US for 50 years. We switched to acrylic lacquer in the '50s. I do recall reading as a boy about a few show-car jobs being done in "real" (nitrocellulose) lacquer, but the material that was available to us, by the time I was old enough to paint a car (mid-'60s) was acrylic lacquer.

Some of the things that have been brought up in this discussion, such as spraying a coat of thinner and spaying multiple coats without sanding, are things I'd forgotten about but which were over my head at the time, anyway. An expert can do things that I wouldn't even attempt. The things I learned about spraying cars, from a car restorer who was quite good, were for the real tyro who wanted a great looking job without having much experience. That was me. Acrylic lacquer would still be my first choice for a job done at home because it dries so fast that you can spray it outdoors (under a tarpaulin, on a windless day, after wetting down the area with a garden hose) and have only a minimum number of insects to sand out; it sands very easily and quickly; you can work so fast with it that you can sand out a screwed-up coat with no real hardship; and so on.

If I were to get serious about it I'd use two-part polyurethane today. But that is very serious business indeed, between the toxicity, the breathing equipment, and the good spraying equipment and expertise required.

-- Ed Huntress

Problems with terminology, I have no idea what acrylic enamel is although it does seem to be available in the UK. We have something called one pack, or used to, which is like coach enamel. It didn't give the finish of cellulose but was tough and very forgiving of poor prep, but took a while to dry fully.

Well I have sprayed cellulose many times coat on coat on the edge of causing runs but not quite in order to get the finish and never had cracking problems.

Terminology again maybe. I started spraying about 1985 and used cellulose and I can still buy cellulose from certain car refinish places although most are running down stocks as it was banned in the UK and maybe Europe 18 months ago for car refinish use. I presume as it's called cellulose it has a nitro cellulose base. I have some old tins of paint, some maybe 40 years old, with titles like cellulose enamel, synthetic enamel and a few other which all look, smell and behave like what I know as cellulose, not very scientific I know, so the names are rather confusing. The older ones seem to have the most potent solvent when you get a wiff of it, one even tops the thinner used for International 2k yacht paint my neighbour uses.

When you say acrylic lacquer is that the same as acrylic enamel mentioned earlier.

I wouldn't consider myself an expert by any means but found that with a bit of practice and playing with the gun set-up to get it right for me I could put the paint on thick and get a great finish straight from the gun and rarely get runs. One of the things I learned early on was that the local mixing scheme cocked up the colour mixing occasionally so I could buy that paint cheap so practicing wasn't expensive. I would do some practice on some spare sheet metal or old panels removed from the vehicle.

Description of the properties of acrylic enamel match my experience with cellulose in the UK although I would leave it for a few days normally to allow full solvent evaporation before flatting so the paint is fully settled or near it.

Yes I agree about 2k paints. I'm not that serious.

Two peoples separated by a common language. d8-) I won't try to untangle that one. If you have an old can of it, the ingredients list probably would straighten it all out.

The material I'm calling enamel goes on much thicker than the lacquer, and it can't be sanded out without sanding off the protective film of resin that floats to the top. The really old enamels (before the mid-'50s or so) contained drying oil rather than plastic resins, and they took a month or more to dry completely. Acrylic lacquer could be waxed in a couple of days, if you allowed it to dry well between coats. Acrylic enamel could be waxed in about two weeks. I imagine it still can, because I think it's still around, along with the acrylic lacquer.

-- Ed Huntress

While the new tins gives details of all the nasty solvents involved the old ones just contain exactly what it says on the tin, ie paint :) , no ingredients list per se .

Need to find a paint expert for this one I guess. Your lacquer description sounds like what I know of cellulose properties where the paint dries by solvent evaporation, the enamel properties sound like what is often called one pack, coach enamel and other names in the UK and while has some solvent to flash off and firm the paint, the main paint binder cures in the presence of oxygen IIRC hence the longer hardening time compared to the purely solvent based lacquer.

That's it. The confusion may come from the fact that the basic resin in car lacquer was changed here in the 1950's from nitrocellulose to an acrylic resin. The behavior of both is very similar, although old-time restorers and show-car people have often said that you could get a higher gloss with the nitrocellulose. I don't know that for a fact, only what others have said.

Enamel was also switched to an acrylic resin base here, a few years later than lacquer. The drying mechanism still appears to be oxidation, as with the older enamels. In any case, it behaves similarly to the old enamels but it dries somewhat quicker. It's still much slower than either type of lacquer.

There also are alkyd enamels but I won't go there, because now it's starting to go over my head.

-- Ed Huntress

I would expect as the paint industry seems to be somewhat International, pardon the pun, these days that if it happened in the US, it happened here also at some point but the colloquial term "cellulose" was maintained generically for a paint with the same basic characteristics but may explain terms such as "synthetic" on the tins. I started using International brand cellulose then the supplier switched to PPG , Pennsylvania Painted Glass IIRC. Most likely the same with enamel. A recent look at the PPG UK site still lists cellulose paint for industrial applications which is still allowed as far as I know.

Heard of them but thats about it. I'll stick to the true enamel as I understand it, ground glass fused to copper or silver substrate so that gets us back to metal working. Must try enameling on some of my spun copper shells, maybe even some cloisonne work.

That's Pittsburgh Plate Glass. Or it was. I think they just use the "PPG" now. They're still a big producer of automotive paints.

The first time I saw the terms "cellulose" and "cellulosing" applied to automotive paint was in a book titled _Sports Car Bodywork_, published in the UK sometime in the early or mid-'60s (I still have the book). Then I saw the term again in some British car magazines. So I suspect that PPG used the term for your market, as it's not a familiar one in the US. That's not to say it isn't used, but it's not the common term.

Do you have any photos to post?

-- Ed Huntress

{...}

Well the shell I had in mind is this

It certainly sounds interesting. My only exposure to doing that kind of enameling was in a high-school shop class. If you get to it and finish the job, post a photo, OK?

-- Ed Huntress

here are some definitions

Enamel - a paint that cures by polymerizing Lacquer - a paint that cures by solvent evaporation - originally from the Lacq (?) beetle

The term lacquer originates from the portuguese word for lac, a type of resin excreted from certain insects[1]. Regardless, in modern usage, lac-based varnishes are refered to as shellac, while lacquer refers to other polymers dissolved in Volatile Organic Compounds (VOCs), such as nitrocellulose and later acrylic compounds dissolved in a solvent generally referred to as lacquer thinner.[2]

While both lacquer and shellac are traditional finishes, lacquer is more durable than shellac.

Nitrocellulose lacquers

Quick-drying solvent-based lacquers that contain nitrocellulose, a resin obtained from the nitration of cotton and other cellulostic materials, were developed in the early 1920s, and extensively used in the automobile industry for 30 years. Prior to their introduction, mass produced automotive finishes were limited in colour, with Japan Black being the fastest drying and thus most popular. General Motors Oakland automobile brand automobile was the first (1923) to introduce one of the new fast drying nitrocelluous lacquers, a bright blue, produced by DuPont under their Duco tradename.

Acrylic lacquers Lacquers using acrylic resin, a synthetic polymer, were developed in the

With respect to paints enamel is a fanciful term, implying that an ordinary latex or oil-based paint has the same properties as true, fired vitreous enamel.

Some enamel paints have been made by adding varnish to oil-based paint.

The term sometimes refers to oil-modified polyesters that were introduced in the early 1930s. The oil is required to stop or enhance the crosslinking of the paint in order to achieve sufficient flexibility of the paint film.

Typically the term "enamel paint" is used to describe oil-based covering products, usually with a significant amount of gloss in them, however recently many latex or water-based paints have adopted the term as well. The term today means "hard surfaced paint" and usually is in reference to paint brands of higher quality, floor coatings of a high gloss finish, or spray paints.

There's actually two sorts of systems called "HVLP", one type uses a dedicated turbine blower and a very large air hose, this is the "true" HVLP. The other types have been retailed as "conversion" guns, they hook up to shop air. The turbine systems are very nice, I've played with them at woodworking shows, very much used in cabinet and furniture finishing. They have virtually NO overspray and can handle heavier water-based finishes like polyurethane. The air hose is about the size of small vacuum cleaner hose, the system runs at very low pressures compared with the conversion guns. They used to start in at about a grand, price has come down over the years and HF has one for a few hundred now. You can lay down stripes of finish almost like they were masked.

The conversion guns have a lot more overspray by comparison, I've got a couple I've used for car painting and wood finishing. HF sells various cap and needle combinations for various materials, a viscosity cup is a must to get best results. The set that comes with the cheapies is usually only suited for really thin finishes, I use it for thinned shellac on woodworking projects. It'll also handle two-part urethane, which is about the only thing you can get anymore for auto finishing around here, lacquer and other solvent systems are dead, dead, dead. Too many VOCs, air pollution greenies are down on them. Price range is a lot greater here, a 50% off coupon at HF brings one down to almost pocket change when they're on sale. Make sure your compressor is up to the challenge, the typical 115v airless isn't going to cut it for any projects much bigger than a couple of feet square.

Stan

Thanks, William.

-- Ed Huntress