Powdercoating question - mild steel

Long story short, I'm welding up a front hitch for my van.  It's all welded up and I want to powerdercoat it before installation.

The mild steel I used is a HF 2" receiver tube that has been notched and welded to a beefy 1/4" x 4x4 angle steel.  The receiver tube cleaned up pretty well because it had been powdercoated from the factory.  The angle is heavily caked with mill scale.  I started with a grinding wheel, but the single-point contact meant lots of grooves, gouges, and tedious.  I switched over to a 40-grit flap wheel on a grinder, and it doesn't even want to touch it.

Does all the mill scale have to come off down to shiny steel before powdercoating?  It doesn't need to look pretty, just max rust prevention.

For best adhesion its generally recommended.  Mill scale us a separate layer of metal and if any moisture fets in there rust can form between the layers (in the position this will be in, unlikely, but possible) and one good hit with a rock can chip the entire layer off.

I've coated stuff with scale on it. It just prints right through the powder.

I don't know if it effects the final durability of the coating, I  think it did it once or twice for parts like what you are working on and just started sand blasting everything. 

Does this Powder coating shop sandblast  the parts before  spraying the powder , 

And does the sprayed powder  get inside a tube to coat it ?

In reply to californiamilleghia :

A shop will probably sand blast. He is doing it himself.

You can shoot the powder into a tube if you take the diffuser off the gun.

Seems like it could be a job for a needle scaler?

Mill scale on hot rolled parts (your 4x4 angle) is very tough. The flap disc you are using will likely polish it very nicely before it ever starts to cut through it. The solution here is a heavy hand with the flap disc. The pressure will help get into the scale and get through it. Once the scale is removed you will want to back off the pressure to keep from marking up the base metal too much. A fresh flap disc with more exposed grit can help too, but as the abrasive wears down the exposed edge does not have much bite. The extra pressure assists in getting to the sharper abrasives. Blasting is the best way, but I don't think you have that setup at the theater.

They make special flap disks just for mill scale.

https://fireballtool.com/products/walter-flexcut-mill-scale-15l843-15l853?srsltid=AfmBOorxje_7fAKME3Srv3r2yP_GyVrVLV-AXYqSarWI6SEC5ZcAS0ZW

The absolute best thing you can do is find someone with a shotblast cabinet and blast it - a lot of our machines are used to remove mill scale prior to coating.  

In reply to stafford1500 :

There are also dedicated flap wheels for mill scale that dont get into the steel underneath. Dark, forbidden knowledge at work there, but they work

Edit:  oop. Missed that^^

californiamilleghia said:

Does this Powder coating shop sandblast  the parts before  spraying the powder , 

And does the sprayed powder  get inside a tube to coat it ?

I'm the powdercoating shop :)  I don't have a sandblaster.

Any powder that gets inside the tube I will blow out with air before baking.  

Well, I went through about 5 flap wheels and I have a combination of polished steel and some leftover polished scale.

I'll see how it coats.  I have to weld on the chain hooks and another support before I can coat.