Priming of steel-tube airframes..

[TwoDeadDogs]

Hi all
can anyone suggest a suitable primer for protecting steel tube after sandblasting? Is Zinc Chromate good enough on its own?
regards
TDD

[poor southerner]

IMHO a polymide cured zinz phosphate 2 pack epoxy from Trimite (Uxbridge).
It's in the MOD spec, with a poly top coat (best to use their SAR-SAP3 chromate primer on the blasted metal( blasted to SA2.5 I hope).

[DubTrub]

poor southerner

You are a person after my own heart! I also use Trimite two-pack epoxy (I have to confess that I start with the two-part etch prime first, but this might be considerd over-kill).

To answer TDD's question: ordinary zinc chromate primer is a single-part coating, and will be removed by both Superseam and Poly-Tack, giving you an unprotected surface.
But over-coat it with any two-pack primer (such as Trimite) then the glue will not present bare metal to the atmosphere.

PM me for a fuller reply, with photos and full description.

DT

[poor southerner]

DT

Yes the SAP/SAR3 is a 2 pack zinc chromate primer, it's the basic yellow one for steel. They do a thicker green one for single coating alu.

But for the sake of your life. when using and 2 pack paints (ie base and isocianate hardner) you MUST wear a full face air fed mask. These are paints are lethal and for proffesional use.

As for Trimite paints. They are on the MOD spec for various systems (def stans) although there are also others, at least when I was involved. But for true corrosion protection I would never rely on anything your local car paint retailer would offer.

[stiknruda]

TDD - I have the parts powder-coated at my local powder-coater.

Impenetrable (virtually) to dope & superseam.

Stik

[javelin]

Stik,

Don't let Donaldson find out if it is on fuselage stuff - the boys at Turweston don't like powder coating 'cos it can mask cracks due to film thickness. Have to agree with you that it does give the best finish though and it may have featured before in one of our productions :E

I use Tetrosyl Autoprime on steel and aluminium, 2 part, non isocyanate, carbon respirator, sticks like the proverbial.

One point, all primers, even the 2 pack ones require a topcoat to provide true corrosion protection because they are designed as a key to the topcoat, not a final finish.

Powder coating does give complete protection.

Finally, to the original poster, make sure that your blaster has some inteligence, uses the correct media and understands that the tubes are thin wall. Also, it really needs priming there, not a day later, not after trailering it elsewhere or the benefit will be lost. I have seen steel corrode 30 minutes after blasting on a humid day.

[TwoDeadDogs]

Hello all
Thanks again to all concerned for the excellent gen.
regards
TDD

[DubTrub]

poor southerner I have been using Trimite systems at work for the last 25 years (is it really that long?). I use the same yellow etch-prime for both steel and ali, on their recommendation.
Your advice about respirators is timely...I am also familiar with isocyanate polyurethane systems... I guess it is good advice to all paint systems, but particularly epoxies and isocyanates.
I would never rely on anything your local car paint retailer would offer

Well I agree that Trimite is the "Industry Standard" in the UK and MOD-approved (there are other paint manufacturers with MOD approval too), but I do also use local car paint top-coats.

On the subject of powder-coating (and indeed stove enamelling), my own opinion (in deference to Stik and his many Pitts friends) is that they are not suitable for aircraft use because:
A) powder-coating is heavy and (as already mentioned) is too flexible to show substrate cracks, and adhesion to the substrate is not guaranteed unless a "wet" prime is applied first. (Another area of our business uses powder-coating, and it is, in my opinion, only appropriate for non-flexing structures that are not in a potentially damp atmosphere.)

B) I respectfully disagree with javelin in that the finish is always slightly "orange-peel" with powder-coating, but this might not be relevant for tubular airframe structures. I also politely disagree with your comment on all primers needing a top-coat...generally, "glazing primers" (i.e. those that form a glaze on the dried surface) do not require a top-coat all the time, but it is of course safer to top coat if weight permits. But I agree with you on the other points.

C) My experience with stove enamelling is that adhesion to the substrate is not guaranteed, despite thorough bead-blasting.

As a conclusion, I would promote the use of wet-paint systems in preference to powder-coating, and also that a light colour is preferable on all welded structural steel members, because it almost obliges the owner to keep things clean, and of course any cracks will show more readily. This is particularly applicable for engine mounts.

Happy spraying!

DT

[javelin]

DT, what a diplomatic, well structured and polite reply !