RTH: Priming bare metal... Grey gotta go? (1 Viewer)

[Jeff Elliott]

Hi All -

Going to epoxy prime my 4x4 Labs bumper today. Have been sanding with 120 grit per the epoxy instructions. All of the metal has a dark grey coating that comes off / turns bright silver when sanded on corners / rounded areas. Question is this: Does that darker grey need to come off / turn silver before I can prime it or is it sufficient to just sand with 120? Pic attached and thanks!

 

[BILT4ME]

Hi All -

Going to epoxy prime my 4x4 Labs bumper today. Have been sanding with 120 grit per the epoxy instructions. All of the metal has a dark grey coating that comes off / turns bright silver when sanded on corners / rounded areas. Question is this: Does that darker grey need to come off / turn silver before I can prime it or is it sufficient to just sand with 120? Pic attached and thanks!

The dark grey is mill scale. The area on the corners has been stretched, welded, or ground so the mill scale is removed easier.

Many coatings are acceptable to apply over well attached mill scale like you have.

However, taking the surface to "white metal" will always give a better adhesion between the coating and metal. This is why companies sand blast or media blast to get the cleanest possible surface. Also, the sanding puts small lines in the material to allow the paint to "bite" into the metal for better contact.

On the can of primer, it should advise minimum surface preparation.
SSPC-SP1 is solvent wipe
SSPC-SP3 is power tool clean
SSPC-SP6 is commercial blast which is a basic blast to remove poorly attached mill scale and to cut an edge into the surface
SSPC-SP10 is near white blast which is to remove all mill scale and make the metal near white. It is the best for paint attaching, but us time consuming and expensive.

 

[fjbj40]

As long as the epoxy you have is DTM and you scuffed as you are and then wash with a solvent the epoxy will stick.

But if time and finances allow then blasting before priming will yield a much better finish, durability and cosmetically wise.

Make sure you color coat after the epoxy has reached its minimum cure time. I have found it’s better to apply finish coat ASAP rather than wait.