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Paint Adhesion Testing Method on Cast Iron/Forged surfaces
Q. We are a gearbox rebuilder in Pennsylvania. We have the gearboxes sandblasted at a sub-contractor prior to disassembly. They are then disassembled and cleaned in a water based parts washing solution with a rust inhibitor added. The gear cases are then dry mag particle checked. They are washed a second time in the same solution. The gear cases are not currently being rinsed after washing. The units are then reassembled. After reassembly, they are filled with oil and have a run test performed. This run test brings the gear case temperature to approximately 140 °F. After run testing, the units are then prepped with a wax and grease remover and primed with an low VOC epoxy primer and top coated with a low VOC polyurethane enamel. The entire process from sandblasting to painting takes approximately three weeks. Some of the exterior surfaces which are painted are rough cast and others are machined areas. The problem we are having is a lack of adhesion of the primer base coat to the gear case, especially in the machined areas. The quality representative for our customer is of the opinion that the moisture in the cast is causing the problem. The water based cleaning solution also seems to be leaving a white residue on the gear case. The quality rep for the customer is recommending heating the units to approximately 125 °F to remove the moisture. Is this a viable solution and can the gear cases be put back in process to be reassembled without priming. Priming of the gear case prior to assembly is not a cost effective solution for our shop.Vic Kirsch
machine shop foreman - Blairsville, Pennsylvania, USA
A. Hi Vic. A primer on bare metal can be an appropriate step for a consumer, but is usually not sufficient/appropriate for factory operations. Bare metal, whether as-cast or sandblasted or machined, should be phosphated before priming. The phosphating not only improves adhesion, but also improves corrosion resistance through a number of different mechanisms.
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey
"Paint & Coating" from AbeBooks
Q. What is the exact method to check the Paint Adhesion on Casting/forging substrates? How is the automobile industry checking the paint adhesion on Casting parts?ARIJIT DAS
Paint Quality Manager - JAMSHEDPUR, India
July 9, 2015 - 2nd Request
Still waiting for the answer. Please help.
- JAMSHEDPUR, JHARKHAND, INDIA