Spray Painting - Tips please

[alan_thomas]

I can happily weld, turn, mill and rebuild anything mechanical, but can I spray paint - No!

I never seem to get the same adhesion to the metal as original paint. I sprayed a kit car a few years back and the finish was great but if you knocked it with a spanner or similar then a chip come come off back to the metal.

I know there are some paint gurus amongst us so what am I doing wrong?What steps and materials are needed to get a good finish AND a hard finish that will take the odd knock.

In the next couple of weeks Iwant to refinish the engine bay while the engine is out and getting the car to a paint shop is not possible. I have a huge compressor with water trap fitted and a fairly cheap gun similar to to pic below. I suspect I am spraying at the wrong pressure, have tried it at various pressures but don't really have an idea what I should be setting the regulator to.

So have youany links or advice on materials or techniquesthat will help me spray paint that will actuallystay there!

- Alan

[dasadrew]

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Can't think of any reason why the paint not sticking to the surface would be connected with the spraying pressure.

Are you stripping back to bare metal, or spraying on top of an existing finish?

If it is the latter, then you need to get rid of ANY trace of grease or oil on the surface but, to be certain, this is not enough. You need to "flatten" the old surface with wet-or-dry or similar and then also completely degrease it with a silicone cleanser.

Trouble is, your new paint system might react with the opened up pores of the old paint, so a neutraliser filler coat should be used first.

Actually, you should never spray paint itself onto anything other than a primer or filler/primer or some other sort of "undercoat"

Looking at your pic, that is a sort of gun that would spray correctly at about 4 bar pressure.

Using 1 pack paints there is a slightly higher chance of getting them to adhere in my experience than a 2-pack.

Whatever way, the surface must be REALLY grease free - not just wiped over with a rag, but chemically clean. The implements for acheiving this are tack rags for the dust and, as said, silicone remover for the grease. (get this from your paint store). Use also sterile vinyl gloves as a fingerprint on the cleaned surface also means a grease spot.

My build up on bare metal is always:

1) adhesion primer (spray from a can does a good job)

2) 2-pack filler/primer

3) 2-pack paint

[Jeff]

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What paint are you using Alan?

[jamcarr2]

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If you are doing it in your garage I would stick with celly I know its not as hard wearing and the finish may not be as good BUT you will live longer with celly even if you wear a good mask 2pack can be absorbed through the skin :shock:

just my opinion

john
Ps, sorry didn't answer your question about adhesion ,what are you flatting down with try 400 before you prime then 600 on the primmer

[Jeff]

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jamcarr2 wrote:

If you are doing it in your garage I would stick with celly I know its not as hard wearing and the finish may not be as good BUT you will live longer with celly even if you wear a good mask 2pack can be absorbed through the skin :shock:

just my opinion

john
Ps, sorry didn't answer your question about adhesion ,what are you flatting down with try 400 before you prime then 600 on the primmer

Spot on! I agree with everything John has said.

Jeff.

[alan_thomas]

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Thanks for replies so far. Cannot remember the details of paint etc from last time but looking for advice before I start this time.

So respraying engine bay I will not be removing all old paint so I assume rub back well, clean off as Dasadrew suggests then a 'filler primer'.

I do wonder if just doing the engine bay I would be better off with 'Rattle cans' rather than the whole compressor / gun setup. Thoughts?

- Alan

[Jeff]

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For awkward areas like that, no air line so spray cans are ok, BUT do not hold a lot of paint(paint thinned down too much) compared to a spray gun and more expensive.

Jeff.

[Jeff]

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If you've got the equipment then use it, far better than cans.

[dasadrew]

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Definitely use the Compressor & Co.

On the safety side, it is always sensible to be, well, sensible!

For two-packs, there are a lot of "scares" about due to the isocyanate content. A lot of the stuff is true, some is false and everyone really has to make up their own mind.

The two hazards of isocyanates are developing asthma on exposure to them (sensitisation) and developing dermatitis following prolonged exposure to skin. Haven't heard of any evidence of any life-threatening or life-shortening effects or cumulative effects in bloodstream. If you drink more than ten pints of beer a week it's probably more life shortening.

The skin contact can be reduced by wearing overalls, gloves, caps and the like.

The breathing of fumes is more difficult and risky if you have an asthma history. All of the regulations are based on occupational spraying at 8 hours a day continuous. In Australia, their HSE even made an exception to the regs for professionals doing small touch-in spraying.

All I can say for your engine bay is how I did my Escort and minimised the hazards.

Use a HVLP spray gun. Works with 2 bar and gives a lot less spray mist.
Mask off and do the four sides in four different sessions. Apart from reducing spray mist build up, this is very useful in avoiding overspray and orange peel on adjacent surfaces and makes it all "manageable".
Definitely don't do any of this in a closed single garage!

For about 15 years I used to be a semi-professional model maker and used resins with isocyanate hardener. The sanding dust got into my eyes continuously and, in my ignorance, I developed conjunctivitis which is mow subsiding gradually over the years. I used to have asthma before I did all the modelmaking and that disappeared as time went on, so the effects of isocyanate dust are definitely not 100% and all-encompassing. This is reflected in the amount of "shoulds" rather than "musts" in the HSE directives.

I am NOT recommending nor endorsing whatever way you choose. The above is simply my way and my experience!

Drew

[dasadrew]

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alan_thomas wrote:

Thanks for replies so far. Cannot remember the details of paint etc from last time but looking for advice before I start this time.

So respraying engine bay I will not be removing all old paint so I assume rub back well, clean off as Dasadrew suggests then a 'filler primer'.

I do wonder if just doing the engine bay I would be better off with 'Rattle cans' rather than the whole compressor / gun setup. Thoughts?

- Alan

If you're roughing up old paint, it is wise to use a stabiliser primer which chemically stabilises the old finish. The old paint could react with your new paint and it might not be immediate, so doing a test patch wouldn't put you on the safe side either.

[alan_thomas]

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Drew I noticed you bought a gravity fed gun, mine is suction feed what are the pros and cons of the 2 types and I notice from Ebay there is a large variation of nozzle size again bigger or smaller? Unlike a professional where time is everything I am happy to sacrifice speed for ease of use.

- Alan

[dasadrew]

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Alan, I'm not sure of the differences between top or bottom paint feed. I think a few years ago someone told me it was just a matter of personal preference. Top cup does mean that every last drop of paint can be used as it all drains down into the jet.

Not sure about needle sizes. I'll check later when I go out to the garage (which has become suddenly snowbound again in the night!)

Drew

[dasadrew]

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Here is a pic of my HVLP spray gun. I am trying to recall which one I used for my Escort engine bay but can't remember. I'll try the HVLP for my next Stag interior bits.

I think if you divide the engine bay into four sides and do them individually the Sata Minijet would suffice.

The needles are 1.4 on the big gun and 1.0 on the MiniJet.

Sata stuff is not cheap, but I have a sort of secret tool fetish......

[Gistan]

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For the bare metal resprays to both of the Stags I restored, I used Acid Etch Primer as first coat on the bare metal, followed by celly primer and top coats.

The usual front end stone chips followed, (celly is softer than 2k ) but none down to bare metal.

No previous spraying experience, or expensive spray guns, just build up several thin coats of colour, then many hours of flatting to lose orange peel.

Even Rolls Royces with state of the art spraying equipment, have the paint work hand polished and flattened.

Gary

[jamcarr2]

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alan_thomas wrote:

Drew I noticed you bought a gravity fed gun, mine is suction feed what are the pros and cons of the 2 types

- Alan

Hi Alan

The main pro for using a gravity feed gun is that it doesn't need such a big compressor ,it works on less CFM .The main con is it can spill paint on the work if not carefully
JOHN