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Around two years ago we installed Galvalume sheets for roofing. As you can see in the images below, the trusses were given only epoxy coating. We were told by the roofing contractor that paint over that coating would not be necessary.

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However, now we are seeing the epoxy coating 'chalking out' i.e., it's just powdering and falling off exposing the metal surface. While we are keen on repainting the trusses, the concern is there is hardly any space between the sheet and upper surface of the trusses. Is there any solution to paint all the sides of the sections without detaching the sheets? Also, would it be essential to remove this existing layer of epoxy for the paint to stick?

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I think the answer to your first question is already evident to you. If you cannot get to it, you cannot paint it. They do manufacture some specialty application tools for tight areas, such as mini-rollers and "paint gloves":

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My experience with these items has been less than fantastic, but maybe getting some paint on those hidden areas is more important than aesthetics.

As for your second question, NO you do not need to remove all the existing paint. You need to remove all the existing paint that is not sound (i.e. likely to peel or flake off) and de-gloss all the remaining areas. This is normally accomplished by mechanical abrasion (sandpaper, steel wool, etc.), unsound paint will come off when subjected to sanding and the rest will be de-glossed. You should thoroughly clean after sanding, and apply a metal primer on bare areas prior to repainting.

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  • Thank you very much, Jimmy. Though I had not specified it, I was wondering if special tools existed for this kind of purposes.
    – Chethan S.
    Commented Nov 28, 2016 at 3:40
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I'm surprised at the chalking. Epoxy generally does that when it's exposed to UV light, such as from the sun. If sun exposure is the reason for the chalking, the impossible-to-access areas also are not getting sunlight and are probably not failing. Because of the UV vulnerability, epoxy is a very bad choice of topcoat for any outdoor painting.

However it's a superb primer when applied over clean steel, and is very compatible with almost any other paint system, including the finicky (but extremely tough) linear polyester urethanes (LPUs) used on jets and boats. As such, it's often supplied as a base coat for parts the supplier expects you to paint later.

A non-epoxy topcoat protects the primer from UV damage.

So I would not remove it unless it's failing. If it's applied over clean near-white metal, it's an excellent primer. I would actually prep it and prime over it (preferably with more epoxy primer which I have in stock) and then my topcoat of choice.

If it's glossy (primer won't be), you'll want to scuff-sand over it with a scotchbrite pad to knock off the gloss. Gloss is a glass-smooth finish that paints cannot physically bond to (nor can dirt, that being the point of it.) A quick scuff-sand creates "tooth" in the surface for the paint to grab onto.

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  • The chalking is not happening everywhere - it's only in lower portions that are exposed to the sun during sometimes of the day. And this coating is not so glossy. Maybe we need not worry much about the top portion then. However, I get concerned due to water drops condensation that takes place under the sheets.
    – Chethan S.
    Commented Nov 28, 2016 at 6:24
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    Healthy epoxy will handle a little condensation OK. Definitely deal with the chalking; it's a fast track to rusting.
    – Harper - Reinstate Monica
    Commented Nov 28, 2016 at 6:41

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