Compressor size and spray painting

[Arron]

I'm writing this post in answer to a question I have seen posted many times on forums - something along the lines of 'I only have a small compressor, can I use it for spray painting?' or 'whats the minimum size compressor I can use for spray painting?'

Many people want to use a small compressor to put to use one they already own, or because they don't want to invest in a large one when their use is infrequent, or like me they are limited to 10 amps and hence 2.5hp.

I've never seen this question answered from experience so here goes.

Now before we go any further I should say the real answer to this question is the bigger the compressor the better - no question of that - so now that I've covered this off there is no need for posts informing us of this fact.

Anyway, I'm currently redecorating our holiday home and this involves sprucing up the kitchen cupboard doors and drawer faces. These are American white oak face-frame jobs. Probably late 80s, some with water damage but most just scuffed and tired looking. Although the kitchen is small it packs in 30 cupboards and drawers, as well as 2 large end boards. I established that they were originally finished with some variant of nitrocellulose lacquer so we decided to just respray them.

I drove up to the beach house to do them last week, taking a couple of middle market (Star HVLP/LVLP, $350 each approx) guns, and having a direct drive 2.5 hp / 50 litre compressor stored up there permanently (cost about $285). Also a drum of precat lacquer and thinners.

Of course when I get there I find I've left the nozzles for the Star guns behind. I didn't want to leave the job for another time because I knew both neighbours were away that day which meant I could spray outside with impunity. Fortunately I had a low-end Supercheap Auto HVLP gun (cost about $60) to fall back on. So that brings in another question I've seen posted here once or twice before - 'are those low end spray guns usable or not?'

Incidently, the largest end-piece was 2200 x 600mm. The biggest cupboard doors were the pantry doors. I decided on one coat on the backs and two coats on the front for everything.

Heres what I learnt:

The obvious limitation with a small compressor is that you get minimal spraying time between long periods of refill. Otherwise, air is air and both a large and small compressor will deliver air at a sufficient poundage to allow any of the guns mentioned to fully atomise their product. The question is how long they will deliver that air for between fills, and that's governed by the size of the tank. The time taken to refill the tank is governed by the size of the motor. A 2.5 hp compressor takes about 36 seconds to fill a 50 litre tank and 21 seconds to drain it to the point where the motor restarts (at least it does when I'm spraying with my usual settings). In that 21 seconds I can put a nice coat of lacquer on a small cupboard door of about 35 x 45 cm but no bigger, so anything larger then that is going to require at least one pause waiting for the tank to refill.

Understand that you cant keep spraying once the tank empties to the point where the motor kicks in because the tank is draining faster then its refilling - at least you can't keep spraying for long.

My main worry with the underpowered compressor was what would happen when you get half way through a door and the air runs out, the motor kicks in and you need to pause to allow it to refill the tank. Would there be a visible flaw where you stop and restart. The answer is it doesnt leave any fault at all, if you are careful and your mixing ratio is appropriate. The pantry doors took about 4 tankfulls to do, but there is no sign of this in the finished job. The finish is fine, its just painfully slow.

Incidently, sometimes I make a pass or two while the tank is refilling, just to keep the edge wet, but I dont think its really necessary.

The only real downer in the job was the big 2200x600 end panel, which was less then perfect. I had to do this in full sun, late afternoon, hurrying because a neighbour had just returned home. The combination of constant start/stop plus the slow advance from the narrow fan (explained below) lead to some streaking. I had to buff extensively with both medium and fine compound the next day to get these out. It finished good, but took 15 minutes of sanding and buffing to get there.

The cheap SCA gun wasn't bad. I found that in order to get a decent pattern I had to keep the fan narrower then I would with a decent gun, as when I broadened it out it broke down quickly. This meant I had to keep both the air and product settings low to match. This seemed to be just a limitation in the machining of the gun, the only way to work around it is to make each pass close together and be very careful not to leave gaps. Nonetheless, even with this limitation I got 3 coats on all 32 doors, drawer fronts and end pieces done within one very long day. The gun didnt give me one splatter, run, drip or drop during the whole day. Orange peel was not evident. Hand fatigue was minimal. Overspray was more then I'm used too - good thing the neighbours were out.

One issue I found with the gun was that, unlike better models, It doesnt have a two stage trigger. You cant pull it half way and shoot out air only. Thats a problem because when I'm doing things like small doors I like to start each one with a full tank, so after finishing a door I shoot away any remaining air in the tank (without loosing any product) and then let it refill while I'm putting aside the door and lining the next one up.

One thing I liked about the SCA gun was that the difference between output when the tank is full and when it is nearly empty is negligible. Oddly, with the Star guns the output drops off perilously in the last few seconds before the motor cycles. I've tried every possible combination of settings to overcome this without success so its strange that the cheap gun scarcely suffers the problem at all.

My conclusion on the SCA gun is that you can get a professional level finish with it, but its limitations mean you have to work slowly and carefully. OK for me but no good for someone trying to make a living. I think you also need a good understanding of how to balance fan, air and product - which ironically only comes with using a range of guns.

I also have one of these SCA guns reserved for sealer and I know that they dont last long before they start going a bit ratty, as you'd expect for the money.

The only real problem I had with the doors was that, because I was painting them above a flat surface, some of the overspray bounced underneath the doors and settled on the backs. This had to be buffed out the next day. I could have avoided this with some type of bed-of-nails surface, which would allow the overspray to fall harmlessly away rather then bounce it back up onto the door.

In conclusion, I think I got a finish at least as good as I would expect from a pro kitchen outfit. I admit its a bit forgiving because as its real timber there is a grain effect - maybe if I was after a mirror finish on flat man-made board I would have been less pleased - I'll never know. Two days later, I examined the faces closely to see if buffing was needed but decided they were more then good enough straight off the gun. The combination of small compressor and cheap gun probably added 30% to the time taken, but didnt compromise the finish on anything except the largest end-board, which suffered more from impatience then anything else.

Half a day to dismantle and power-sand, one very long day spraying, then half a day to reassemble and tweak. Very worthwhile .

hope this helps someone
cheers
Arron

[Scally]

Thanks Arron, very helpful as I have been considering replacing an old compressor.
I can't justify a more expensive compressor than the one you described. Although the Belt drive version comes on special occasionally and I could go for it. I'm an not sure if it is worth the extra money?
The specs on the 2.5hp compressors always say suitable for nail guns and spray painting.
With your explanation I have a better understanding of their limitations and can decide if would suit me.
As I would only use a compressor for occasionally with Nail guns and for spray painting, it would be suitable.

Cheers

[Arron]

Thanks Arron, very helpful as I have been considering replacing an old compressor.
I can't justify a more expensive compressor than the one you described. Although the Belt drive version comes on special occasionally and I could go for it. I'm an not sure if it is worth the extra money?
Cheers

I can clarify the issue of direct drive vs belt drive to some degree. I have both direct drive and belt drive 2.5hp / 50 litre compressors running alongside each other.

The belt drive is this one Sydney Tools - Dari 50L 2.5HP Direct Drive Air Compressor - 50285M . It seems it now $249 from Sydney tools. I bought it late last year.

The direct drive one I cant find a web image for but it cost about $700 when I purchased it 2 years ago.

Putting the two alongside each other, and using a stopwatch, I find that both are pretty similar in performance. Both take about 36 seconds to fill (by 'fill' I mean to go from the point when the motor switches on to the point when the motor switches off). The pressure difference between the upper and lower points is the same in both machines, so therefore both should deliver exactly the same amount of air between fills. I have confirmed this by fitting the same hose and gun to both, and yep, its 21 seconds.

This is a little disappointing because when I bought the direct drive one I was told that it was a 'high spec Italian made' thing that I was buying.

Incidently, I talk about the time taken to fill and to empty because they are the only measures that matter to me, and I measure them with a stopwatch. All the quoted performance/capacity figures are apparently so tainted by marketing hype that they cant be relied on. You can work the real capacity/air delivery figures out from the information available to you as an owner, there are plenty of websites that explain how to do it.

The next issue that matters to me is noise. The direct drive makes a horrible clangy noise which is much louder then the belt drive. I'll take a decibel meter out and measure them if anyone wants to know exactly. In our noise sensitive neighbourhood I always feel edgy when using the direct drive machine.

Then there is duty cycle. I cant comment on this because I rarely run either hard enough for it to be an issue.

Then there is length of service. I have only had both about 2 years so cant comment on this.

The fittings on both machines are similar. However the cut off switch did fail on the belt drive machine when it was one month out of warranty, leading to overheating and a gasket failure. The water filter on the direct drive machine seems to work better. I always use an additional water filter attached to the gun and it is quite dry after using it on the direct drive machine. It has about 5mm of water in it after using it on the belt drive.

I think that if you are an occasional user its quite sensible to regard a compressor as a commodity - in other words, buy a cheap one, flog it, and expect to replace it. You should get 10 years of occasional use out of even the cheapest compressor, and at the end you still have an electric motor and a tank to use for other things.

Incidently, using 32mm C1 type nails, I get about 100 nails per tank on these compressors.

cheers
Arron