Mipa AK 105-20 single pack zinc phosphate primer

[Lomax]

Looking for the cheapest way to prep my steel cabin roof for painting with Epifanes Multiforte, and I found the Mipa AK 105-20 zinc phosphate primer, which is a single pack primer costing just £5/litre. This compares very favourably with for example Jotun Pilot QD at around £8/L (also a single pack zinc phosphate primer). But I've not been able to find any opinions or reviews of the Mipa primer, so I'm a little hesitant. Too good to be true? I'm also wondering whether I can get away with using a car body filler (polyester resin) for fairing between coats, or if a "marine" filler (epoxy resin) is required to get a lasting result?

Grateful for your thoughts!

 

[GHA]

2 part epoxy all the way! Hempadur 45141 is epoxy of choice at the moment.
Everything else will come back to haunt you... (my experience anyway)
Also important is no bolt holes, welded studs & no wood touching painted steel anywhere, welded stainless tabs or spacers seem to be working well on here.
Lasts for ages & ages then

 

[Lomax]

2 part epoxy all the way! Hempadur 45141 is epoxy of choice at the moment.
Everything else will come back to haunt you... (my experience anyway)
Also important is no bolt holes, welded studs & no wood touching painted steel anywhere, welded stainless tabs or spacers seem to be working well on here.
Lasts for ages & ages then

I've found Valspar EP401 at £10/L, which is probably as cheap as it gets for an anti-corrosion epoxy primer. Still twice the price of the Mipa single pack. The roof has paint on it already, which I don't intend to remove completely, only sand down, and I'm wondering if an epoxy primer might do bad things to it? I don't know what primer was originally used, but the topcoat is definitely Epifanes Multiforte, which is also what I intend to use (already have 5L of this).

 

[Lomax]

I bought the Valspar epoxy primer - seems a decent product and I got it for £45 delivered, which is really cheap at just £9/L. I will test it on any spots where I go down to the steel when prepping the roof, and check if it reacts negatively with the existing paint. Any leftovers will be used for repainting the hull on my fibreglass dinghy...