I believe you didn't apply the epoxy layer before the steel returned to room temperature, at least near 30 Celcius. If that was the case, the problem is that. Do wait until it's cold enough before you try next time. If not, make sure your epoxy (if two parts) is mixed slowly without entrapping air bubbles in it. And if one part, make sure it is homogenous. Also, do apply enough thickness on the steel plate. I've used many epoxies on steel and this is never the case if you ensure you omit the three things that I just mentioned. Good Luck!

@Malsha. The room was Air conditioned and the substrate was cold.

But I think I can do the other two things; try to mix the two parts slowly and also apply enough thickness. Do you think 200 micrometer is too less ? I am using film applicator for applying the coating.

dear Muralishwara Kakunje ;

in my opinion, this case is caused by the impurities in the substrate .did you clean it well? the substrate should be uniform roughness maybe you need chemical itching before treatment.

Mahmood Sir, I used abrasive papers to prepare the specimen and have also degreased with acetone.

For me it looks like uneven surface treatment of steel. Not coated spots should have opposite surface properties - hydrophobic or hydrophilic, depending on the nature of the epoxy resin.

Mahmood H. Ievgen Obraztsov I used a handheld motorized abrasive pad to finish the surface followed by polishing. I got a clean surface with even roughness without any mill scale, grease, etc. Now the output is better.

Thanks for your inputs

with pleasure

Dear Muralishwara Kakunje,

Before the resin thickens and cures it needs to prepare bubble out by prepared torch. Use a heat gun on its low heat levels and aim the gun parallel to the bubbled surface, make sure you are not aiming directly straight down on the bubbles, the heat gun should always be at an angle so that empty spaces can be avoided. If impurities in the substrate and proper polishing not done properly secondary cause empty space cannot be ignored.

Hope it work out.

Ashish