Compressor size and spray painting
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 :U.
hope this helps someone
cheers
Arron
