Ken, I don't believe I said that solvent penetration will cause a lifting p=
roblem. The problem I refer to is swelling which is commonly referred to in=
the autobody industry as buttholing. This swelling is usually not noticed =
until all of the solvent has left the paint film(often more than a month af=
ter painting). Areas of old paint which have been feather sanded to a smoot=
h taper(say a large chip or scratch that has been sanded out) tend to swell=
with the application of solvents then shrink back as the solvent leaves th=
e film. So your primer layer is applied(which contains solvent) and it cure=
s to its workable state. The swelling can be evident at this stage as the p=
rimer is sprayed and dries. Surface sanding of the primer will remove the s=
well temporarily and everything can look fine through final topcoat applica=
tion but the areas which were swollen and planed out during the guide coat =
block sanding stage can shrink later leaving a butthole in the shiny smooth=
hard earned surface. Epoxy primer can help to prevent this by bonding to t=
he old finish and sealing it off from solvent and moisture penetration whil=
e also providing an ideal surface to apply further topcoats to. Moisture=
penetration has been known to cause problems with highly porus substrates =
like fiberglass and the panels of the gmc which are sheet molded compound o=
r smc. If there are bare areas of smc that are sanded wet or bare areas whi=
ch are only primed then sanded wet later(wet blocking of 2k primer) and wa=
ter is absorbed into the panel it can become trapped under subsequent paint=
films and eventually force its way to the surface in the form of small bli=
sters usually after being in the hot sun. Although I have never witnessed t=
his type of failure first hand I have seen it after the fact under the orig=
inal finish on most of the smc panels on my coach. Application of a sealer =
coat prior to subsequent topcoats will prevent moisture migration. Acid =
wash or similar chemical conversion coating is the best way to obtain a ch=
emical and mechanical "tooth" on most bare metals for painting. However it =
can create problems when used on a partially bare panel as the acid can sof=
ten and become trapped in the remaining film. I believe that is why the man=
ufacturers direct that it is to be used on completely bare metal panels onl=
y(there might have been advances in this area that I am unaware of since we=
have not done restoration work at my shop for 10+ years) If the oxid=
ation layer is removed from the aluminum immediately prior to application, =
epoxy primer bonds well. We repair cars every day with aluminum panels and =
and use epoxy as a sealer prior to surfacers and topcoats. We guarantee our=
work for life so we obviously only want to do it once. We have never had a=
n adhesion failure on an aluminum panel that we have repaired. All petro=
leum coatings contain solvent. The solvent has two purposes Allow the =
material to be transferred to the substrate(spray) Allow the material to =
flow out across the substrate After the transfer and flow out residual so=
lvent remains in the film until it can evaporate away. This process varies =
with heat/air movement/ film thickness If you want a graphic visual of h=
ow much of a paint film shrinks, pour off a small quantity of any mixed 2k =
paint and put up on a shelf and watch how much it shrinks over time after i=
nitial cure. These residual solvents can swell old finishes and shrink back=
long after the topcoat is applied. There are many different beliefs =
on painting processes and materials. I have been using PPG topcoats and all=
different types of surfacers for 20+ years. Most of the technical info I h=
ave just spewed out here came directly from ppg training certification sess=
ions. I hope some of this can be helpful to those looking to refinish their=
coach. Do with it what you like Sully 77 royale
Sent from my Verizon Wireless BlackBerry
-----Original Message-----
From: Ken Wolkens
Sender: gmclist-bounces
Date: Mon, 08 Aug 2011 07:16:12=20
To:
Reply-To: gmclist
Subject: Re: [GMCnet] Repainting a 1973 GMC Photo Log
For small areas of bare aluminum I've always prepped with a metal prep acid=
wash then shot the urethane primer. I've done numerous lower units on boa=
t racing (100 mph+) engines this way and have never had an adhesion related=
failure. I've also painted entire aluminum body panels this way ( Jaguars=
). Years later the paint is still sticking.
I've found the key to successful aluminum adhesion is a finish of 400 grit =
and the removal of the layer of oxidation on it's surface. Without the etc=
h, no primer in the world will stick.
Since 99% of the aluminum on my coach is still covered by the original prim=
er and paint I'm not worried.
As far as urethane primer allowing a lift, it's highly unlikely. That prob=
lem is usually only an issue with laquer and enamel primers like the ones i=
n a spray can. The way to test for this potential issue is to take a cloth=
and soak it with laquer thinner and wipe your primer. If you can wipe off=
the primer then you have a problem. Since the urethane primer is a cataly=
zed polymer, the solvents won't touch it. As far as permeation goes, if th=
e solvents are on the primer long enough to get through, then you will have=
plenty of runs and sags to worry about anyhow. FWIW, I've sprayed numerou=
s full vehicles with every type of old paint below this primer without one =
lift issue.
With regard to adhesion properties (at the polymer level) the list from low=
est to highest are:
Polyester
Epoxy
Urethane
Urethane always wins.
I have used many gallons this primer for over 10 years without failure on e=
very type of substrate that I can imagine like:
2x2 twill Carbon Fiber
Kevlar
Steel
Cast Iron
Machined aluminum
Cast aluminum
Fiber glass panel
Various flexible plastics (with an adhesion promoter)
So with all of this said, that's why I chose to use a 2k Urethane Primer.
--=20
1973 GMC 23'
All Birch and Maple Interior Cabinetry. TZE033V100221
"The Honeycomb Hideout"
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
roblem. The problem I refer to is swelling which is commonly referred to in=
the autobody industry as buttholing. This swelling is usually not noticed =
until all of the solvent has left the paint film(often more than a month af=
ter painting). Areas of old paint which have been feather sanded to a smoot=
h taper(say a large chip or scratch that has been sanded out) tend to swell=
with the application of solvents then shrink back as the solvent leaves th=
e film. So your primer layer is applied(which contains solvent) and it cure=
s to its workable state. The swelling can be evident at this stage as the p=
rimer is sprayed and dries. Surface sanding of the primer will remove the s=
well temporarily and everything can look fine through final topcoat applica=
tion but the areas which were swollen and planed out during the guide coat =
block sanding stage can shrink later leaving a butthole in the shiny smooth=
hard earned surface. Epoxy primer can help to prevent this by bonding to t=
he old finish and sealing it off from solvent and moisture penetration whil=
e also providing an ideal surface to apply further topcoats to. Moisture=
penetration has been known to cause problems with highly porus substrates =
like fiberglass and the panels of the gmc which are sheet molded compound o=
r smc. If there are bare areas of smc that are sanded wet or bare areas whi=
ch are only primed then sanded wet later(wet blocking of 2k primer) and wa=
ter is absorbed into the panel it can become trapped under subsequent paint=
films and eventually force its way to the surface in the form of small bli=
sters usually after being in the hot sun. Although I have never witnessed t=
his type of failure first hand I have seen it after the fact under the orig=
inal finish on most of the smc panels on my coach. Application of a sealer =
coat prior to subsequent topcoats will prevent moisture migration. Acid =
wash or similar chemical conversion coating is the best way to obtain a ch=
emical and mechanical "tooth" on most bare metals for painting. However it =
can create problems when used on a partially bare panel as the acid can sof=
ten and become trapped in the remaining film. I believe that is why the man=
ufacturers direct that it is to be used on completely bare metal panels onl=
y(there might have been advances in this area that I am unaware of since we=
have not done restoration work at my shop for 10+ years) If the oxid=
ation layer is removed from the aluminum immediately prior to application, =
epoxy primer bonds well. We repair cars every day with aluminum panels and =
and use epoxy as a sealer prior to surfacers and topcoats. We guarantee our=
work for life so we obviously only want to do it once. We have never had a=
n adhesion failure on an aluminum panel that we have repaired. All petro=
leum coatings contain solvent. The solvent has two purposes Allow the =
material to be transferred to the substrate(spray) Allow the material to =
flow out across the substrate After the transfer and flow out residual so=
lvent remains in the film until it can evaporate away. This process varies =
with heat/air movement/ film thickness If you want a graphic visual of h=
ow much of a paint film shrinks, pour off a small quantity of any mixed 2k =
paint and put up on a shelf and watch how much it shrinks over time after i=
nitial cure. These residual solvents can swell old finishes and shrink back=
long after the topcoat is applied. There are many different beliefs =
on painting processes and materials. I have been using PPG topcoats and all=
different types of surfacers for 20+ years. Most of the technical info I h=
ave just spewed out here came directly from ppg training certification sess=
ions. I hope some of this can be helpful to those looking to refinish their=
coach. Do with it what you like Sully 77 royale
Sent from my Verizon Wireless BlackBerry
-----Original Message-----
From: Ken Wolkens
Sender: gmclist-bounces
Date: Mon, 08 Aug 2011 07:16:12=20
To:
Reply-To: gmclist
Subject: Re: [GMCnet] Repainting a 1973 GMC Photo Log
For small areas of bare aluminum I've always prepped with a metal prep acid=
wash then shot the urethane primer. I've done numerous lower units on boa=
t racing (100 mph+) engines this way and have never had an adhesion related=
failure. I've also painted entire aluminum body panels this way ( Jaguars=
). Years later the paint is still sticking.
I've found the key to successful aluminum adhesion is a finish of 400 grit =
and the removal of the layer of oxidation on it's surface. Without the etc=
h, no primer in the world will stick.
Since 99% of the aluminum on my coach is still covered by the original prim=
er and paint I'm not worried.
As far as urethane primer allowing a lift, it's highly unlikely. That prob=
lem is usually only an issue with laquer and enamel primers like the ones i=
n a spray can. The way to test for this potential issue is to take a cloth=
and soak it with laquer thinner and wipe your primer. If you can wipe off=
the primer then you have a problem. Since the urethane primer is a cataly=
zed polymer, the solvents won't touch it. As far as permeation goes, if th=
e solvents are on the primer long enough to get through, then you will have=
plenty of runs and sags to worry about anyhow. FWIW, I've sprayed numerou=
s full vehicles with every type of old paint below this primer without one =
lift issue.
With regard to adhesion properties (at the polymer level) the list from low=
est to highest are:
Polyester
Epoxy
Urethane
Urethane always wins.
I have used many gallons this primer for over 10 years without failure on e=
very type of substrate that I can imagine like:
2x2 twill Carbon Fiber
Kevlar
Steel
Cast Iron
Machined aluminum
Cast aluminum
Fiber glass panel
Various flexible plastics (with an adhesion promoter)
So with all of this said, that's why I chose to use a 2k Urethane Primer.
--=20
1973 GMC 23'
All Birch and Maple Interior Cabinetry. TZE033V100221
"The Honeycomb Hideout"
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
