Hi and thanks to Malcolm

[treblesbassman]

Hi all
New to the forum. Done quite a lot of spraying and have always been plagued by occasional tiger striping on large panels in my finish coat. Always looks fine wet but when it dries the stripes appear

I am always very careful to spray at 90 degrees to the job, 50% overlap, gun pressure etc. Just wanted to thank Malcolm for his tutorial about setting the fan control - it is the best desciption I could find on the web!

Great site and thanks to all

 

[malcolm]

Welcome. Glad the page was handy.

I painted my first couple of cars with one of those sprayguns that comes with lots of other air tools in a kit. That didn't atomise properly which causes a similar distribution problem. Got completely fed up with it one day when there was no window between orange peel and overspray and replaced it.

 

[jaykay]

it could be stripping in the primer,if it was bad and sprayed over it would show up in final finish.
make sure you dont under cut on primer rub down/flatting.

 

[treblesbassman]

Thanks for your responses guys.

I didn't appreciate it was the distribution of paint within the fan that the control adjusted - rather than the fan width itself. I have good kit - Devilbiss GTi, big compressor, airfed mask and shop made spray room - so I am pretty sure the problem is down to me! (I used to have the el cheapo Clarke kit as well Malcolm - the Divilbiss atomises rally well). On my gun the same seems to happen - opening the fan control from fully closed very quickly produces a full width spray - long before the control is fully open. So it must be the paint distribution rather than the fan width it is altering beyond a certain point. I have pressure set at the gun when triggered at the recommended 30 psi.

I am currently lacquering a large furniture piece with 10% sheen ac lacquer and having the striping problem. I have improved it by overlapping slightly more than 50% but there are still one or two stripes just visible at certain viewing and light angles. I think perhaps I was slightly out with the overlap here and there - it does seem to be critical you get the overlap correct at all times (not surprising really I suppose!)

I had set the spray control about half a turn back from fully open but I am now going to experiment with fully open setting. It does seem my spray pattern was centre-heavy the way I had it.

I have never had any issues with dry spray, orange peel, fisheye or the myriad of other problems - just this tiger striping.

You are right Jaykay, thanks - On the largest panel I was trying to get rid of the stripes by overspraying with thinned lacquer. Looked fine when wet but the stripes just kept showing through once it dried. In the end I had to flat back through several coats to remove the stripes but it is still not perfect.

This is getting expensive on lacquer!

I will be experimenting some more with wider fan settings and overlap today

Cheers guys

 

[jaykay]

ok try breaking the rules. up pressure to 40 psi.(everyone i know over drives the hvlp guns.
if that starts to come in to what you want. widen the fan just a tab more.
still not right,add a touch more thinners.
lets know how it goes.
being dry on your over laps,sounds like it could do with a bit more thinners.
what lacquer/paint are you using?????

 

[steveo3002]

you could try after your last coat of basecoat...let it start to flash a little , whack the pressure up, fan wide open and stand back a little ...maybe 18-24" or so

waft the gun about randomly ..flicking your wrist so the paint sort of floats in the air and just lands on the panel

would be so easy to demo in person....anyway its called a drop coat , some colours/ paints need it

might also be that you are putting the clear on before the base is fully dried

 

[treblesbassman]

Thanks for the suggestions jaykay and steve - I've sorted it (I think - anyway tiger striping has gone and the finish is even - same low sheen all over). I am ashamed to have to report the gun wasn't set up properly. I didn't do the basics - i.e. checking for good atomisation, correct fan shape for aircap and even fluid distribution. I was wrong about it being centre heavy - the fan was that squeezed it looked like a bar bell - spray pattern was starved in the middle and I didn't notice it. It did spray wet in the middle but much more at the edges just where I was overlapping the strokes - compounding the problem. The fan setting is now only a 1/4 turn from closed which seems a bit odd but it works. The fan opens immediately you turn the control (as Malcolm said) on this GTi (compliant, not HVLP) so I am still getting a reasonable width. But now the lacquer is evenly distributed across the fan.

I was also laying it on far too heavy which also made the whole thing worse. Found out coverage should be 75gms per m2 - I must have been laying at least 60% more than that in a coat!

So I lowered the fluid delivery a bit and backed the pressure slightly as there was now a little less fluid to atomise(still getting good atomisation though) and sprayed a much lighter coat by sweeping nearly twice as fast. It looks flat and is smooth and drying to the correct sheen level. Its Morrells 330 range acid catalysed melamine lacquer 10% sheen - 5% max thinners recommended which I have kept to.

I realise now that I have always had a tendency to lay on lacquer too thick - but I have gotten away with it in the past. So I have learned a lot from this little exercise - namely to make sure to set the gun up for the finish properly and check wet mil thickness - or at the least correct coverage!

So now my next problem is to sort out the dodgy end grain in places. Workpiece is a lectern made of 30mm thick Valchromat, which is through-dyed mdf, in this case anthracite colour. Looks medium grey before finishing but a clear finish turns it dark grey with a slight greenish tint - sounds bad but looks nice - and the flakes of the mdf show through the finish, giving some pearlescence (spelling?). The lacquer is not wetting out the end grain evenly and there are light coloured patches here and there. Not many but not perfect. I think (hope) sanding will sort it. I appreciate this is not a woodworking forum - but thanks for your input guys!

p.s. I like the sound of the drop coat steve and good description - a bit like feathering but all over the piece and from an increased distance? - be a good one for me to experiment with in the future