- Aerosols and a brush will paint an engine well. A gun earns its place on a bare block on a stand, or when the paint only comes in a tin.
- A full-size gun wants about 280 litres of air a minute. Most hobby compressors can't supply that for long.
- A mini gun uses far less air and suits engine parts better than a full-size one.
- Water in the air line ruins more finishes than poor technique. Fit a trap.
- A gun makes far more mist than an aerosol. With one-pack paint that means a proper filter mask. With 2K it means air-fed breathing apparatus.
Do you need one
Often you don't. One-pack enamel from an aerosol, or POR-15 from a brush, will give a finish most people can't tell from sprayed. The application guide covers both.
A gun is worth having in three cases. The first is a bare block on a stand, where a gun lays an even coat over a large, awkward shape faster than cans will. The second is paint that comes only in tins, which includes most of the marque colours in the shop. The third is that you already own one for bodywork. Professional restorers spray engines for all three reasons, and because they have the booth and the breathing apparatus that two-pack paint demands.
Which paints go through a gun
POR-15 says its Engine Enamel can be thinned by up to 5% with POR-15's own reducer and sprayed through an HVLP gun. Eastwood sells its Ceramic Engine Paint to be brushed or sprayed. Frost's own 500 ml engine enamels are brushing paints, and Frost says they can be thinned "up to 10% with White Spirit for application with a Spray Gun".
Follow the maker's thinner and the maker's ratio. Enamel thinned with the wrong solvent can curdle in the cup, or dry with no gloss.
Two-pack paint is made for a gun, and the two-pack page explains what it asks of you. U-POL's data sheet for its S2025 primer gives a typical professional set-up: an HVLP gravity gun with a 1.6 to 2.0 mm tip at 1.8 to 2.0 bar.
Gun types and tip sizes
Nearly every gun sold for car work today is gravity fed, with the cup on top. The labels HVLP, LVLP and "compliant" all describe guns designed to put more of the paint on the part and less into the air than the old high-pressure guns did. HVLP does it with a large volume of air at low pressure. LVLP and compliant guns aim for a similar result using less air, which matters if your compressor is small.
The tip size decides what the gun can spray. As a guide, taken from how DeVilbiss's UK dealers specify the FLG5:
| Tip | Use |
|---|---|
| 1.3 mm | Thin topcoats and clears |
| 1.4 mm | General topcoats, including thinned engine enamel |
| 1.8 mm | Primers and heavy-bodied paint |
For an engine, a 1.4 mm tip covers most jobs. Buy a 1.8 mm gun as well only if you will be spraying high-build primer.
A mini or touch-up gun deserves more attention than it gets. It holds about a tenth of a litre, throws a narrow fan, and uses a fraction of the air. On rocker covers, brackets, pulleys and the tight corners of a block it is easier to control than a full-size gun, and it wastes less paint into the air.
The air behind it
This is where most home set-ups fall short.
A DeVilbiss FLG5 is a typical mid-priced full-size gun. Its dealers quote an air consumption of 277 litres a minute, or 9.8 cubic feet, at 2 bar at the gun inlet. The compressor has to deliver that continuously for as long as you hold the trigger.
Compressor listings usually quote the air the pump draws in. What comes out, called free air delivery, is lower. If a listing gives only one figure, assume it's the intake. A 50 litre direct-drive compressor drawing 200 to 250 litres a minute will run a mini gun comfortably. With a full-size gun the tank empties in seconds, the pressure sags, and the finish turns dry and rough. For a full-size gun, look for a twin-cylinder machine drawing 370 litres a minute or more, and expect to pause between passes while it catches up.
The tank size mostly decides how long you can spray before the pump cuts in. The pump decides whether it can keep up at all.
Keep the air dry
Compressing air squeezes the water out of it, and that water collects in the tank and the hose. When a drop reaches the gun it lands in the wet paint as a crater or a blister.
Fit a filter and regulator with a water trap at the compressor or on the wall, and drain the tank before each session. A small inline trap at the gun inlet catches what gets past. Keep the hose long enough for the air to cool before it reaches the gun, because warm air carries its water straight through a trap.
Set the pressure at the gun with the trigger pulled, since that is where the maker's figure applies.
Cleaning the gun
Clean the gun as soon as you finish. Paint that dries in the air cap or the fluid passages spoils the fan pattern next time.
For one-pack paint, run the maker's thinner through the gun, strip the air cap and tip, and clean them with brushes. Never poke wire through the air cap holes. Do it outdoors or with the same ventilation and mask you sprayed with, because a gun spraying thinners makes mist too.
After two-pack paint the rules tighten. HSE's guidance sheet MR3 says guns "contain isocyanate residues" and should be cleaned "using only dedicated and enclosed gun washing equipment that has local exhaust ventilation". Where that isn't possible, it says not to spray washings in the open workshop, and to do it in the booth wearing air-fed breathing apparatus.
See the spray guns, compressors and water traps in the shop.
Sources
- POR-15 Engine Enamel directions: thinning with up to 5% reducer for HVLP
- Frost BMC Green Engine Enamel: brush on, or thin up to 10% with white spirit to spray
- Eastwood High Temp Ceramic Engine Paint listing: brush or spray
- U-POL S2025 High Build Primer data sheet: gun type, tip size and pressure for a 2K primer
- DeVilbiss FLG5 at Spray Guns Direct: air consumption, inlet pressure, tip sizes and uses
- HSE MR3: cleaning two-pack paint spray guns: enclosed gun washing, where washings may be sprayed