Cold Rolled/Hot Rolled prep for powder Coat

[njschaben]

I am looking to buy more steel for an upcoming project of about 100 2'x2' signs. Up till now I have just been using hot rolled sheets and then putting them in muriatic acid to remove the mill scale, then sending them to someone else to powder coat. I am in the process of ordering materials to build my own 4x8 oven to do the powder coating myself.
Here are my questions:
1. Is the muriatic acid enough to clean and etch the steel or does it still have to be sandblasted after the acid?
2. Do I need to apply any iron phosphate or something before I powder coat?
3. Do I have to out gas Hot rolled or cold rolled steel?
4. If I buy cold rolled steel can I just clean the oil off with something and then powder coat or does it have to still be etched some how to get the powder to stick better?

I love how well the cold rolled sheets cut and I really love that I don't have to put it in the acid tank. I have just never powder coated before and I don't know if it is too smooth for the powder to stick to? Any advice on which steel you prefer and what steps you take to prep and coat the steel would be appreciated. My closest powder coater is 100 miles away I just want to do it myself, but I don't want to have to sandblast anything. I will only be coating new steel sheets for my own projects. Thanks for any advice.

[Gamelord]

Regardless if it is hot rolled or cold rolled, the steel will have oils and coatings on it that will not give you a good adhesion with powder. The only way I have been able to get the powder to stick right and look right is to either grind the metal down with a sander or grinder to bare metal, or muriatic acid then sand blasting. Sand blasting is easiest and quickest and gives the most uniform look. After prep, I metal etch everything, preheat to gas out the metal and then coat.

Powder coat can reproduce a fingerprint, so if your piece is not perfect, your powder will not be perfect either.

[njschaben]

Thanks Gamelord. What do you use for a metal etch? I see PBTP has CRYSCOAT 2707 IRON PHOSPHATE, and eastwood has Eastwood After Blast - Metal Prep. Would one of these be best or do you use something else? Also when you out gas do you just get the metal up to 400 then pull it out? Or do you bake it at a different temp for a certain amount of time? thanks.

[AnotherDano]

Cold-rolled is my choice. When powder coating, all I do is run over the entire piece with a wire brush cup in an angle grinder for dross removal and a uniform scuffing. My workspace for this step is dedicated for this, with no oily or waxy work done in the same area.

Before coating, the piece is sprayed down with denatured alcohol and scrutinized for 'breaks' in the flow which indicates crap on the metal. It also takes care of oils from my fingers. Disposable gloves are always used with this step to avoid oils from the skin. The pieces are air-dried and just before coating, any 'fuzzies' are burned off with a propane torch. Faraday areas are torched last and the powder is shot there first.

The denatured alcohol is expensive ($15.00/gal where I am) but re-do's are even more costly. An industrial solvent pressure spray unit is worth the investment, as it provides a finer spray that saves the alcohol. Be mindful of the soot that may remain in the kerf-width only cuts. When I have a lot of these thin cuts, I may use acetone to cut through it and wash it out. That soot can result in edge-pull and poor adherence in the kerf cut.

This setup has proven itself to be indispensable for grinding/brushing. Use a thick and flat steel base. Cam clamps are from the world of the woodworkers. (Pardon the mess).

IMG_0139.JPG

[AnotherDano]

Thanks Gamelord. What do you use for a metal etch? I see PBTP has CRYSCOAT 2707 IRON PHOSPHATE, and eastwood has Eastwood After Blast - Metal Prep. Would one of these be best or do you use something else? Also when you out gas do you just get the metal up to 400 then pull it out? Or do you bake it at a different temp for a certain amount of time? thanks.

Outgassing is used to get soaked-in contaminates in cast materials. Oils and junk will migrate to the surface from the minute voids in the cast surface. Most coaters will 'cook' the piece at 450~500 degrees for an hour or so, then blast, blow-off and coat. Sheet/plate steel does not need outgassing. Don't confuse outgassing should not be confused with a 'burn-off' step that may be required for nasty, oily pieces to be coated. Not usually associated with the stuff we do; Think 'engine parts'.

The sub-base prep products are used primarily for things that will be subjected to heavy use in nasty environments and probable hazards like off-road ATVs and saltwater.

[Gamelord]

I use the metal etch from Eastwood, it is working great for me so far. Outgassing: see above comments, pretty much spot on. I pre-bake everything before coating just as a precaution, takes a little time and costs a little more for electrical/oven heating costs, but as mentioned, it is a lot less than having to redo a piece.

Not sure why you are against sand blasting, since I got my blasting cabinet I have been in heaven. I am actually in the process of purchasing a larger one that can do full 4' pieces (58" wide).

Hope that helps.

[plain ol Bill]

I'll throw in my 2 cents worth here too. Blasting leaves a "profile" that gives the powder an excellent surface to grab onto.

[njschaben]

Thanks everyone. I am going to order some CR-16 gauge and HR-14 gauge since I can't get 14 gauge in CR. I still have to build my oven and then I will just have to try some pieces. I will take a few pieces down the street and sand blast them and see what the differences are. I know the powder coater I used on my first set of signs just powder coated directly over my bare metal signs that were just put in my muriatic acid tank. They seemed to turn out just fine without being sandblasted, but I don't know if they will just not last as many years or what. I am not against sand blasting I just didn't know if the muriatic acid etched the metal the same as sandblasting. I will just have to do some trial and error when I get my setup installed. Thanks for all the advice and ideas.

[AnotherDano]

Ask if they have PO; pickled & oiled.
That's HR with the mill scale chemically removed, then oiled to avoid rust.
It should be priced between HR and CR.