Manual Airbrush Compressor

N.B I have no connection with any supplier or manufacturers mentioned they just happen to be who I used.
Humbrol/Badger make or at least sell basic spray guns / airbrushes and a small grit blaster.
The grit blaster wouldn't work with the propellant cans as they released liquefied propellant due to the volume needed which clumps the grit and stops it working, this also made the cost prohibitive. You can buy a small compressor for air brushes but they are expensive (ok you can get a cheap one for £60 now)
Humbrol/Badger sell and so do others an adaptor for 1/4 bsp to the small airline used with airbrushes. They also sell an adaptor to use the propellant cans regulator on a car tyre.
Now if you are using a foot pump car tyres take a lot of getting up to pressure to start with. Even a small one is quite bulky ok if you've got a shed. But is your partner or your mother going to let you keep it in your bedroom or the living room?
The other problem when the pressure drops you've got to stop and pump it up again ok with grit blasting but not with a paint mix that needs using before it sets. It is quite straightforward to fit a second valve to a wheel rim so you can keep it up to pressure with a foot pump as you spray but it is still bulky what is needed is a smaller air tank

A 2 litre PET(PolyEster Tubing) fizzy drink bottle.
Yes I am serious. The idea to use one came from an article that utilised one as the air reservoir on a model aircraft to power the retractable undercarriage this was working at 300 psi as I only need 50psi and pressure tested to 100 psi. The lack of anything more complex than a drill in the way of tools and not having a gas cylinder or fire extinguisher to modify is what made this an attractive proposition.

2 screw fit schrader valve stems.
If you have access to dies to cut threads you can make these from the brass cores of snap fit tyre valves. I've seen an instructable that used snap fit vales directly in PET bottles but I've not tried this and it may only have been in the lid.
A small section rubber sheet. A section of cycle inner tube is ideal.
Schrader to propellant can regulator adaptor the part number is, badger #59-029 they range in price vastly for the same item from £6 to over £30 so shop around.
In looking for that part I've found other adaptors that do the same job but remove the need for the original propellant can adaptor as well. I have no experience with them so whilst it looks like one should work with this set up I don't know that it will.

Start by drilling the smallest hole you can through the central moulding pip on the base of the bottle, measure the valve stem select the drill / drills you are going to use I prefer to use a spur point wood working bits as the outer teeth cut the diameter and the central spur locates in the small pilot hole and in my opinion give a smoother hole in the relatively thin material, if using standard twist drills it is best to open up the pilot hole in small steps. I also found it easier to do this with the drill bit mounted in a removable chuck and turn it by hand.

Drill the bottle lid in the same manner using the central moulding pip as your centre point

A bit of old bicycle inner tube is best for this/

Start by cutting the top off another bottle use this to mark the circle you need on the rubber you need to draw around the inside of the neck onto the rubber a biro will work I found an ultra fine point marker pen was better, cut out with scissors on or inside the as this disc needs to be a good fit inside the central ring of the bottle cap.

Hold the disc in place and mark through the hole you have drilled for the valve stem remove the central disc using a craft knife.

A square the same size as the bottle cap with a central hole for the valve stem is adequate for the bottom valve.

The cap is the easy one fit the washer you made, push the stem through the hole in the cap fit the outer seal (this came with my screw fit valves) the spreader washer and nut, tighten the nut.

The base of the bottle is not so easy. You need a stick longer than the bottle and a good fit inside the vale stem.

Fit the rubber seal to the valve and push it through the neck of the bottle, fit the stick inside the valve stem and use it to guide it through the hole in the bottom of he bottle, I found it easier to hold the stick still and move the bottle, fit the seal spreader washer and nut, tighten it up.

PET bottle aren't intended to be used like this.
Pressure testing is a must.

Fit the bottle top and screw down tight, check the valve cores are fitted correctly
Attach a pump and pump air into the bottle until it is just becoming firm. This should only take a few of strokes. Disconnect the pump and check for leaks. stick it in a bucket of water and look for streams of bubbles, tightening up anything that needs it
Reconnect the airline a foot pump with a built in dial gauge is best for this and final use although a stirrup pump type would work just as well place the bottle the other side of a fence or wall to you or at least around a corner you don't want to be hit by the blast if it does fail, ear defenders and goggles wouldn't go amiss either.

The required working pressure is 50psi twice working pressure is a reasonably standard safety test so inflate it to 100psi

The standard regulator for the propellant cans (normally comes with the basic spray gun set) has a standard thread the regulator needle just screws in to open the valve. The adaptor a commercial part (badger #59-029) is a simple brass ring threaded to EN417 on the outside Schrader thread on the inside with a rubber sealing ring, the existing "O" ring in the regulator needs to be removed first then the adaptor just screws into the regulator and on to the valve stem.

The needle of the regulator then screws in to press on the valve core and open it well that's the theory. In practice I've found you can't fit the regulator with the needle in place as it opens the schrader valve before it is seated.You can fit the regulator without the needle then fit the needle but then the valve opens before the "o" ring on the neck of the needle seals, it does go in far enough to seal still. You have 3 choices.

Use it like this and remove the needle completely each time you wish to shut off the pressure,this risks losing the needle.

Remove the valve core from the end where you attach the regulator, this means you are using the valve on the air brush etc as the sole control and can't swap over with out depressurising the whole set up.

Modify the needle,this means cutting about 3 to 4mm off its length, down side you can't use it with propellant cans any more If you have more than one regulator this isn't a problem, BUT you have to get it right remove too much and it won't work at all.(see additional steps)

Safety note,

From the above it is evident that ordinary butane, propane or mixed butane&propane canisters with an EN417 fitting could be used as propellant cans even though both gases are used as propellant in other aerosol products since the withdraw of CFCs its probably not a good idea to do so for prolonged periods.

Another advantage of making your own small compressor reservoir.

Attach the regulator to one valve, your pump to the other preferably a foot pump or stirrup pump with a built in dial gauge to the other, you can judge it without a pressure gauge just by your paint flow, but a dial gauge makes it easier. You could get by with a basic bicycle pump but that would be probably best done by an assistant, in fact having an assistant to do the pumping if you are doing fine air brush work might be advantageous any way.

Connect your airline to the regulator pump up to 50 psi and use your mini grit/soda blaster, spray gun or air brush as usual, depending on your pump a stroke every second or two is all it need to maintain pressure and I found it easy to empty a 50cc spray jar with out getting short of breath.

Fit the adaptor and the regulator to a schrader valve with out the needle and you will find it sits on the end of the vale core before the threads engage.

Measure the distance between the top of the regulator body and the underside of the thumb screw of the needle.

Screw of the regulator the needle in until the 'O' ring has just disappeared into the neck of the regulator remeasure the distance.

calculate the difference, it should be almost the same as the distance from the top of the neck of the regulator to the top of the 'O' ring, you could just eyeball that distance but get it wrong and its useless.

Mark this distance on the end of the needle cut it off slightly over length so you still have metal to remove with swiss/needle files, preferably use a jewellers saw blade 35 to 70 TPI (www.proopsbrothers.com/jewellers-piercing-saw-blad) a small hack saw or you might just get away with a good pair of side cutters.

deburr with a swiss file check for length you should be able to screw it in and feel it touch the valve core part of the 'o' ring should still be visible file a little off the end and recheck repeat until the 'o' ring is completely inside the neck

This isn't really need,it is quite functional as it is but it doesn't hurt to supply it with a bit of protection. Rubbing around on hard surfaces causes scuffing when not inflated the bottle is very flimsy and easily crushed both of which are potential sources of weakness, and it should be replaced if showing signs of damage. Supporting it by the valve stems is also probably not the best way to do this but it is the easiest a cradle to support the bottle would be better.

I've simply used an off cut of 18mm chipboard flooring slightly wider than the bottle cut to the same length as the distance between the valve stem securing nuts. and a couple of bits of hard board with an 8mm hole drilled 75mm up from the base to take the valve stem. one of these was screwed to one end of the base board, as it happend the securing nut on the bottom is actually flush with the base of the bottle so this was the end I put through first, the other end plate was then passed over the top valve stem and screwed to the base board.

This is a remake of something I first did some 30+ years ago. It didn't need much technical ability, I made my own valve stems or at least modified some myself the first time.

This time I brought them before I found the ones I'd made in the past. £6 for a pair of valve stems plus another £6 for an adaptor makes this cheap compared to the £60 for the cheapest airbrush compressor I was able to find even if you need to buy a pump as well its still a big saving, but if you wanted to power it up you could do.

Electric tyre pumps can now be had for about the same price as a foot pump or stirrup pump, around £10.( so allow for a power supply as well and your half way to the cost of a commercial item)

The one I have in my car now can be set to shut off at any pressure from 5 to 100 PSI. But that is it it shuts off, it won't restart until the power is switched of and on again, I took a look inside, a pressure switch that trips a relay via a transistor to cut the power to the pump and keep itself powered in that state until the power is disconnected completely buy the operator, it could probably be rewired to do that on its own with a simple time delay circuit or a second pressure switch to reactivate it when the pressure has dropped say 5PSI. For now this is something I have no need of but I've included my ideas for others to consider.