polyurethane clear coat...what´s happening?

[dschwartz]

Hi all.. i have a question for the paint experts..

i like polyurethane clearcoats for its durability and resistance, but i´m having siome troubles.. i put 2 or 3 coats and let it dry overnight..
then, in the morning i apply the last coat before going to work..but that last coat bubbles up!!! WTF???

any ideas why?

[dano12]

Lots of variables there, I would assume temperature could be causing it. Do you do the other non-bubbly coats in the morning? Constant temp or humidity?

If you can get a toaster oven for pedal baking, I'd highly recommend it--you get pretty much consistent temperature at each drying stage.

Also I've found for the last coats, they need to be thinner than previous. Generally by using less poly. And going back over the brush strokes very carefully to remove bubbles.

[dschwartz]

brush strokes? i´m using spray..

the previous coats were made at noon, it´s pretty warm here at noon, like 30°C..
the last coaty was made this morning.., maybe 13°C..and humid..probably water micro condensation caused the bubbles and wrinkling..

[dano12]

brush strokes? i´m using spray..

the previous coats were made at noon, it´s pretty warm here at noon, like 30°C..
the last coaty was made this morning.., maybe 13°C..and humid..probably water micro condensation caused the bubbles and wrinkling..

Yeah, I would suspect different temps and humidity as the problem then.

[R.G.]

Humidity is a persistent problem to many painting systems.

But you may need to let more time pass for the undercoats to get fully hardened and solvent free, or low-temp bake the solvents and water out. Baking at temps that do not degrade the paint speeds up solvent evaporation and chemical hardening. However, it makes things worse if you put a few coats on with inadequate hardening/evaporation, then bake a last coat. The sealed-in lower coats gas off and cause bubbles.

[Mark Hammer]

Imagine a pair of pants that are too big, and a belt that is too tight.  The belt is essentially "gathering up" the layer underneath it.  When the underlying layers are not fully cured/dried and the upper layer contracts toomuch, you get the same thing happening - the lower layers bunch up just like the pants would.

[frank_p]

John Lyons, told me the respect the drying time suggested by the manufacturer and in some cases it could take about 72 hours between each layer of polyurethane so the layers that are under the one that you are painting is hard an off not to be "gathered" by the ones you are putting on.

http://www.diystompboxes.com/smfforum/index.php?topic=72185.msg583349#msg583349

Bought some new paint not long ago but did not continue because it's starting to be cold here (even got our first layer of snow this night and think that Mr Hammer must be really happy of that).

If you are getting tired of painting projects that goes wrong, try to buy paints (primer, base coats, clears) that come from the same company.  Usually they don't want customers that are going angry after their products.  So they must put some R & D and efforts at mixing their recipes so the different "parts" are compatible, and you don't have too much troubles with their brand from one end to the other of your painting process.

Now I will really have to finish my little painting booth with that weather...   

Good "luck" Daniel.

[Mark Hammer]

Bought some new paint not long ago but did not continue because it's starting to be cold here (even got our first layer of snow this night and think that Mr Hammer must be really happy of that).

No.  I'm not.  Every Canadian Tire tells me they are booked up solid for changing regular to winter tires.

[frank_p]

Bought some new paint not long ago but did not continue because it's starting to be cold here (even got our first layer of snow this night and think that Mr Hammer must be really happy of that).

No.  I'm not.  Every Canadian Tire tells me they are booked up solid for changing regular to winter tires.

That was an antiphrasis Mark, I know: you've complained all winter long last year...  Me too, I really hope that the winter will be more mild this winter...

[dschwartz]

well i´m praying to not get toasted this summer..santiago has a very dry and hot summer..perfect for painting boxes.. but nasty for your health (specially with the kind of air pollution here)

[Paul Marossy]

I really have come to dislike polyurethane clear coats a lot. a few years ago, I painted a couple of things white and then put a polyurethane clear coat on them, and over time, they turned very yellow. Very annoying. 

[frank_p]

I really have come to dislike polyurethane clear coats a lot. a few years ago, I painted a couple of things white and then put a polyurethane clear coat on them, and over time, they turned very yellow. Very annoying. 

Do you remember what brand Paul ?
So you use no clears at all ?
I've been told that polyurethanes that are made for outside use are yellowing less with time (because of some additives like anti-U.V.)...

[GibsonGM]

Yellowing is sort of a 'normal' part of polyurethane...exterior grade will do it less, of course.  That's what gives older furniture (and white strats) its antique patina.  Spray clear coats might be a better way to go for pedals (Krylon Crystal Clear or the like). 

As detailed above, a proper curing time between coats is essential, and low-temp 'baking' can help.  It also doesn't hurt to hit a dry coat with some very fine steel wool, like 4-0, between coats.  Just a little, don't take it off, but scuff any rough stuff off and then clean any bits of wool off.  I'd go with something like 4 coats.  Too many coats may = peeling.   Of course, the Krylon won't get you that thick, gleaming poly coat, but it will hold up very well....you could try using a gloss base paint (little wool on that before clear coat or it might not adhere) to get a more luscious looking final product...

~Mike (I'm a painter)

[dschwartz]

well that yelow gives a vintage look, isnt it?

i also have acrylic clear coat, but i find it too fragile and it tends to peel off sometimes.
I like poly, it´s hard and doesnt react with the paint below..

[CodeMonk]

I bake each coat in this little oven I made (I haven't baked anything for awhile. Not since before all those damn leaves fell):
Maintains temp at 150 - 155 degrees.
This works VERY well. And I use thin coats. Colored coats I have baked for up to 1 hour.
Actually, I baked a colored coat for about 3 hours once (Forgot I had it in there).
Did the same thing, although only for about an hour with a clear coat and it turned yellow
So the clear coats I keep it down to 15 minutes otherwise it yellows.
I have been using a engine enamel I got at Kragen (Auto parts store) for the clear coat.







The material on the lid is some insulated stuff I got from a fabric store.






frank_p, I never thought about that before (Buying your colored, primer, clear from same manufacturer. Damn good idea.