Peeling Powder Questions and Answers

Question:

We are an auto ancillary unit, catering to various automotive OEMs in India. We are brazing En1A , B.S.970 bushes to HLGP (High Aluminum Galvanized Skin Passed) steel material sheets of I.F. quality. The jobs are subsequently powder coated using epoxy grade of powders. After complete powder coating and its baking at specified temperatures, the powder coating peels off in and around they brazed area by just rubbing by hand. We do not face similar problems in the case of M.I.G welding of the similar bushes on the same jobs. We tried two different types of brazing rods but the problem is persistent. We are approaching you as to yours suggestions for arresting this peeling of powder coating. 

Answer:

I am not familiar with the materials you mentioned (BS = British Standard?). In general, brazing requires a fluxing action to break down the oxides present in order to get the two materials to bond once the braze filler metal has become molten. This flux residue usually should be removed since it is not good for paint adhesion and, if left on the part for a long period of time, could cause corrosion problems. The easiest thing to try would be simply quenching the part in clean water after brazing to shock off any flux residue. If this is not effective enough, going to an acid cleaning step may be necessary to remove the flux residue.

Question:

We found powder peeling off parts. After talking to the shop supervisor he informed me that he had to put a second application of powder on some parts. We are using polyester, could you tell us the best way to recoat over powder? 

Answer:

To maximize intercoat adhesion on a two-coat powder coating several steps should be taken:

First, ensure that there are no additives in the powder that can be an impediment to adhesion (i.e. slip additives, gloss reducers, etc.). Some powder formulations cannot be re-coated without severe adhesion failure.

Second, scuff sand the surface between coats for a good bond. Be careful to remove any sanding dust.

Third, do not allow the powder coated part to be pretreated by the washer a second time, as this will cause spotting and other problems. Rinsing with fresh water is perfectly acceptable.

Lastly, ensure that both coats are fully cured.