Wash Primer vs. Sandblasting

Question:

My facility repairs Marine Corps equipment such as tracked vehicles (i.e. tanks, armored vehicles, engineering equipment). We use paint products such as wash primers. The tech data sheet informs us to use the wash primers to improve paint adhesion by etching the metal substrate. Most of our equipment has already undergone sandblasting, which roughens the surface for good paint adhesion. I believe we are unnecessarily applying a wash primer after sandblasting. We should be able to go directly to the epoxy primer and then apply the finish coat. Could you provide some input on whether my conclusions are correct? R.T.

Answer:

Unfortunately, your conclusions are incorrect. While it is true that sandblasting and wash primers are both pretreatments, it is also true that the degree of corrosion protection provided by these systems is markedly different. Indeed, the two-component wash primer does etch the metal. It also forms an organometallic complex at the primer-metal interface that increases the adhesion of the wash primer. Furthermore, the increased adhesion provides improved corrosion resistance by reducing undercutting of the paint film in a damaged area. It is this phenomenon that improves salt spray resistance.

On the other hand, sandblasting does provide a lesser degree of corrosion protection by providing "tooth" for adhesion of a primer. However, it does not provide the reduced undercutting provided by a wash primer. This is the reason the finishing specifications call for use of wash primers where extreme corrosion protection is required. Furthermore, it is inconsistent with the spit and polish of the U.S. Marine Corps to go around in rusty vehicles.

From a technical standpoint, you would be better off eliminating the sandblasting, but I suppose you do that to strip the old paint. The other thing is, from a legal standpoint, you could be in trouble if vehicle failure due to rusting results in a tactical problem. I recommend you apply the wash primer and comply with the specifications.