Spray guns - nozzle size

[jaspr]

I've read and reread all of the useful stuff on this forum concerning use of spray guns. One of the problems I found is that it is not always apparent whether people are talking about suction or gravity feed guns.

So, when I came across the following table, I thought it answered a lot of my questions about spray nozzles. Maybe it will help others too. It's taken from an article by Jeff Jewitt simply called "Spraying" in FWW's "Finishing and refinishing furniture", Sep 2006. And if I put it here, I'll be able to find it myself later.

10-15 sec in Ford No. 4 cup*

(water = 10 sec)

Gravity feed - 1.1 mm nozzle
Suction Feed - 1.3-1.4 mm nozzle
Pressure Feed - 0.7mm nozzle

15-23 sec in Ford No 4 cup

Gravity - 1.2-1.3 mm
Suction - 1.5 mm
Pressure - 0.8-1.0 mm

23-35 sec in Ford No. 4 cup

Gravity - 1.5 mm
Suction - 1.7 mm
Pressure - 1.1 mm

35-40 sec in No. 4 cup

Gravity - 1.5-1.7 mm
Suction - 1.9 mm
Pressure - 1.1-1.2 mm

40-45 sec in No 4 cup

Gravity -1.7 mm
Suction - 2.0 mm
Pressure - 1.2-1.3 mm

45-55 sec in No. 4 cup

Gravity - 1.9 mm
Suction - 2.2 mm
Pressure - 1.3-1.5 mm

55+ sec in No 4 cup

Gravity - 2.2 mm
Suction - N/R
Pressure - 1.5-1.7 mm

*Viscosity time using a Ford no. 4 cup (fill, raise 150mm, time when first break in fluid stream occurs.)

I'm just hoping that the column formatting works - otherwise it will be a mess!

[MacS]

Jaspr,

That's an excellent post, its worth saving.

Thanks,

MacS

[durwood]

The chart you have supplied does show that gravity feed compared to suction feed compared to pressure feed nozzle size is consistant because of the different way the materials are delivered from the gun.

A gravity feed has the advantage that the paint will naturally flow out of the nozzle because of the effect of gravity.

Suction feed requires the air pressure to make a vacuum at the nozzle tip and draw up the material from the pot under the nozzle so a bigger nozzle is needed as the material will be dried out more on the way to the surface by the air than a gravety feed.

Pressure boosts the amount leaving the nozzle so a smaller one is called for.

What it doesn't show is that for the same guns there would also be a vast difference in the size and number of holes in the air cap.

The gravity feed gun will probably have smaller and less holes than the suction feed and the pressure feed will have a lot more than the suction feed.

BUT it then depends on what the actual material is that you are spraying.

If it is fast drying the nozzle needs to be bigger and the air amount needs to be different. Same goes for slow drying the air cap will be drilled to allow differently to balance the material.

When a paint is produced the manufacturer determines the viscosity that gives the most optimum result. He then forwards the product on to the gun manufacturer who tests out various combinations of nozzle and air cap sizes. Sometimes he will start with a nozzle size ( large for high viscosity and small for low viscosity) and then produce an air cap that allows the paint to go on with the best result. If the air holes are too many (or too large) the paint will be dried out too much and the result will be sandy dry paint or orange peel or lumpy when it is sprayed. If they are too small and not enough air is supplied the paint isn't atomised properly and the finish is also lumpy not smooth and wet enought to flow out into a flat glossy finish.

What you must do when purchasing a gun is to determine what paint you are going to spray. If you buy one just on nozzle size it may not spray the paint you intend using anywhere as good as you need to spray it.

If its an enamel and the "Set up" ( the nozzle, air cap and needle) and its wrong you can't get the smooth glossy finish you need to have as the paint is not designed to rubbed back and compound/polished. Or you may have one which makes it difficult to paint without getting runs and sags.

Or you might be buying a gun that uses far too much air for the compressor you have so its impossible to paint an area due to its size before you run out of air.

The viscosity reading you have shown for water also applies to virtually all the solvents, thinners and reducers used to thin paint all take about 10 sec. Likewise all normal finishes are thinned to about 20 seconds in a Ford 4 cup, if its enamel you need to add around 10% thinner if its lacquer its 60% thinner, if its polyurethane its whatever the manufacturer states to get the mixture to 20 seconds. If you have a paint and there are no instructions you add thinner till it goes through the cup in 20 sec and you will be able to spray it satisfactory - if you have a gun set up for that type of paint. If the temperature is hot or cold the amount of solvent will vary (less when hot more when cold) but always 20 seconds.

In a lot of cases the amount of thinner is not very critical, if I add 60% thinner to lacquer it will thin it from about 3 minutes to 20 sec if I double the amount of thinner it drops to about 17 sec, if I add the same again it goes to about 14 sec. So as long as you initially add the correct amount it works, adding extra only means you get less lacquer each coat as there is more solvent in the mix which evaporates off. As you haver made it so thin you can correct this by holding the gun further away which will dry out the excess thinner.

Enamel on the other hand is fussy, add 10% you get the 20 sec add 15% and it drops to about 15 sec and as the solvent is so slow drying you have a runny mess.

So please ask the spray gun seller to show you the chart for the gun you wish to purchase which gives the code numbers for the material . If you want a gravity ( or suction feed etc) know what capacity your compressor is and get one suitable for the capacity of the compressor for that paint not just a 1.4mm nozzle or a 1.8 or whatever.

[jaspr]

Thanks heaps Durwood - I always look forward to your very informative posts on spraying. I learn heaps every time.

In my case, I have a reasonable sized compressor (10 or 14 cfm, depending on where/how you read it) and a choice of suction and gravity guns - the latter with two different sized spray 'set ups'.

The biggest problem I have is working out when to use which. You've given me a few more pointers.

thanks