Industrial Paint Question

Question

I am having a problem with my industrial painter, and I can't seem to figure out what it is. I have castings (cast iron) that are primered by the foundry with "Checkrust Shopcoat Primer" - AKP-363 Red. They are then machined by my company, and washed. Next they go to my painter where they are sanded, wiped down with thinner, and the particulates blown off. Then another coat of primer is applied - "Devguard 4160" in Red. Then the topcoat of "Centari" Acrylic Enamel is applied - in a variety of colors.

My problem is adhesion. The topcoat, along with the 2nd layer of primer is peeling off, literally in sheets, with hardly any effort at all.

At first I thought it was a preparation issue – my painter could have been taking shortcuts, but the more I investigate, the less likely I believe that to be.

My next thought was curing time for the 2nd primer layer. So we ran an experiment: One casting was primered and allowed to cure for over 48 hours before applying the topcoat. Another casting was primered and allowed to cure for less than 2 hours before applying the topcoat. The results were the same – both units peeled extensively.

When the paint peels, it is bringing the 2nd layer of primer along with it; leaving the original layer of primer nearly intact.

My thought is this: either the 2nd layer of primer is not adhering properly with the 1st layer of primer (a chemical incompatibility?): or the solvents in the paint are penetrating the 2nd layer of primer, causing it to separate from the 1st layer of primer.

Can anyone point me in the right direction? How do I find basic knowledge on paint: hardeners, accelerators, etc. Where can I find basic knowledge on paint types and compatibility: enamels versus urethanes, versus polyurethanes, etc?

I am a novice in regards to paint types or chemicals used, so any help that can be offered would be greatly appreciated.

Answer 1

I have direct experience with 5 part "system" painting and priming industrial steel products.

I believe you may find that the SANDING is NOT being cleaned off enough. Sanding may produce a surface which though wiped and blown off retains particles lower than the average surface. NOTHING will adhere when there remains ANY PARTICULATES.

Castings are difficult enough, but a red shop prime is weak followed by the rest.

Answer 2

It looks like the second coat which is an acrylic enamel was applied after the primer has already completely hardened hence there is a poor adhesion between the two types of paint. What could have been done if the primer has already hardened is to apply a mist coat ( about 25 microns of devguard) over the primer then apply the second coat while the mist coat is still tucky.

Answer 3

Have a talk with your coatings vendor.

Answer 4

Sounds perplexing, after reading I though of two areas where the problem could stem from.  

• Pre-treatment:  You stated that you “wash” the part once you receive it.  I assume this is a standard degreaser or a 3 or more stage washer.  Often inter-coat adhesion problems stem from residue being left on the substrate.  Start your investigation there.

• Second coat of Primer:  This may sound simple but why prime the part for a second time.  The base coat should give you ample rust protection.  Have you tried top coating the shop coat primer?  If you are worried about durability you could switch to a 2 component system that would save you a step and increase color and gloss life.

I have used and recommend a line of paint called “Next Gen” from Diamond Vogel Paints.  It has a “wet on wet” technology that allows you to prime the part then immediately top coat the part, that could solve the problem and speed up you line time.