Humidity while painting

[Skybolt]

Hi Jeff.
Make sure the water is out of the previouse coat (the one that has blushed) before hitting it again later. Could have later peeling / adhesion problems.

Why do you say that Aerothane is a big pain?
What is the waiting?
Why is there sanding between coats?

Cheers Eric.

[Dave S]

The partial explanation as to why chilled paint produces a glossier finish is - the lower temp reduces the evaporation rate of the solvents in the paint giving the paint a bit more time to flow out and run together before it sets up. Also - to some extent - this will may slow the evaporation rate to help deal with elevated humidity - hopefully keeping the surface of the paint from chilling below the dew point and possibly avoiding the sugar frosted flake deal.

The general principal is anything that reduces the evaporation rate of the solvents in the paint will have the effect of producing a glossier finish - within limits; such as, lower temp of the paint, lower ambient temp, slower evaporating dilution solvent (which is the reason for the availability thinners of different evaporation rates) coarser spray droplets (which can have other bad effects if taken to extremes), holding the gun closer to the surface - all of these things will have an effect of giving the paint longer to run together before it sets up. Then consider the opposite happens if a person goes the other way i.e. higher temps, smaller spray droplets, faster evaporating solvent & holding the gun further away from the surface.

There is a ton more to the issue when dealing with humidity - such as quick solvent evaporation can easily chill the surface below the dew point where the end result can resemble a sugar frosted flake because of the moisture condensation on the surface of the paint. Mainly this is dealt with by using additives like retarders along with slower evaporating solvents.

Urethane behaves differently than polytone (which acts a lot like lacquer).

When a person considers all of the variables like gun air pressure, flow rate of the paint through the gun, air temp, paint temp, viscosity of the paint, solvent, how close the gun is held , different paint types - it is almost enough to make a person's head hurt thinking about it. Annnnnnd......I probably forgot about a dozen other variables in the process too.

What always seems to work best for a starting point is the paint manufacturers instruction - then do some trials and experimenting - as much as it is all supposed to be science - there seems to be some art and voodoo involved with painting too.

Dave S
St Paul, MN

[Skybolt]

Quote from Dave ..[What always seems to work best for a starting point is the paint manufacturers instruction - then do some trials and experimenting - as much as it is all supposed to be science - there seems to be some art and voodoo involved with painting too.

Dave S
St Paul, MN] end quote.

Dave you are correct.
Start off with the instructions from paint manufacturer.
Try to understand what is going on around the process. If you have never touched a spray gun do some trials before anything else.

Maybe try this...stop by a friendly car repair facility. (a small independant owner shop is your best bet.) Introduce yourself and explain you are about to spray paint for the first time on an aircraft.
Offer him $100 (in twenties) In return for a little eduction with urethane paints and the application of. If you too guys hit it off good at the greeting, he will be your best friend during the painting. And if he introduces you to his spray technician for advice; you are on a roll.
PS. Dont go in there on a Monday morning, probably early afternoon Friday is your best time.

I have been in the car refinishing paint business for over 40 years. Reading the tech data sheets is very important. But these do not teach you to paint, The same thing, if you purchased a welder and read the books; will you be able to weld?
As in welding and painting, there is an understanding (voodoo) and art involved. And there is also a challenge involved if you want to do it, but think of the satisfaction....thats one of the reasons we build airplanes

Eric.

[DanB]

I'm back at it after too many setbacks to mention. The only one I thought would be worthy of posting here is a costly lesson I have learned. Probably not unlike many of you, I tend to be on the meticulous end of the building spectrum. The old addage of analisis paralysis fits me and has taken it's toll in the painting department. I have tried to do things the right way by purchasing "quality" equipment such as my Binks HVLP. I found it on Craigs list which was my first mistake. I thought i was getting a great deal when I paid $150 for a $300 gun. As it turned out, either the air cap or the needle valve (or both) was dammaged. I'm not interested in anyone trying to help me analize the problem because frankly Scarlett , I don't give a damn.
I know if I purchased it new this post would not be here. After many "hair-pulling" sessions, I took the advice I was given at the beginning of my painting and went down to Harbour Freight and bought the $40 HVLP they have (Central Pneumatic Model 66222). This morning I walked out to the booth, mixed the paint, put it in the gun, did a few test sprays and away I went. It was like night and day. I put the paint down like I have been doing it all my life.
Yes Virginia, there is a spray gun Santa Clause and he lives at Harbour Freight.

[Slyfox]

Well, I think it really doesn't matter to much on the gun, but the person spraying, to a point anyway. I will say, humidity while painting, the heading on this post is very very important. Spray when it's over 50percent and you might as well throw the paint down the toilet. I've wasted many of paint this winter wanting to get r done and having the humidity up, just plain wasted the day and the paint. Do yourself a favor and look on the http://weather.noaa.gov/weather/current/KSFF.html for your airports local weather. Just fill in your airports identifier. In it is the relative humidity. Very usefull. My paint jobs come out when it's around 35 or below. If it's never that low where you are, you will need to get a hold of the paint manufacture and figure out how to paint in higher.

[Lion8]

Paint related question. I have alot of experience painting autos,not airplanes. Are there any problems repainting a Kitfox? Do you strip off the paint? (Weight consideration?)

[Dave S]

Hi Tom,

If a person is repainting a fabric airplane - my suggestion would be to determine what the airplane is covered and painted with already and then check the manufacturer of cover/paint system for advice. If it is covered with the polyfiber - they have an excellent book about using their products which I would recommend - the book includes topics on repair/touchup and repainting. For poly fiber - it matters if you have the polytone (which behaves much like lacquer and very easy to renew/repair/refinish) or Aerothane (which is their two part urethane). Since you have worked with Auto paint - you have a leg up on the deal. From what I can tell - the primary difference between auto and aircraft paint systems is aircraft systems have less "fillers" in the formulation - go on a little thinner and are more resistant to cracking - shouldn't use auto urethane on fabric airplanes - I am told.

Theoretically, polytone could be ragged down with solvent and dissolved off the aircraft down to the fabric so a person can sort of start from the base coat - certainly a stinky outside job. Can't do that with aerothane - (maybe I shouldn't say can't - but I believe stripping aerothane would involve more pain and suffering than I ever wish to endure).

We went with aerothane; and, I have decided that 20 years from now when the fabric should be replaced is the next time I will even consider painting it. On a car, a solid layer of urethane, properly watersanded and cleaned up is an excellent base for a new coat - on a fabric airplane with urethane - I'd be reluctant to try it because of the thickness of the final paint and my (warranted or not) paranoia about a surface with as much flex as fabric possibly cracking with many layers - but I am sure there is a way and I am sure someone has done it. Again - finding the manual for the paint system and talking to a knowlegeable distributor about it really helps. Got lots of tips from my poly fiber rep back here.

Sincerely,

Dave S