What to use to fill holes on wood before PMF?

So I’m going to hopefully start my PMF over my Overland Style Teardrop trailer but have a few questions first. If you have followed any of my threads you know this has been somewhat of a fiasco for me so I want to make sure I get this right. I’ve got the trailer skinned with 1/4” Birch Plywood and am trying to decide if I should try using “The Mix” before doing the PMF or just go straight onto the untreated plywood?
Also I’m trying to figure out a different way to do my bottom edge of the PMF. When I built this I should have made it so my bottom of my sides cam down and contacted the metal frame but I have a 1/2” piece of Baltic Birch on the bottom and my sides only come down to the tops of those sheets. I’d like to figure out a way to extend the PMF maybe clear down on my Frame rails. I’m not sure if PMF would stick to metal but I was thinking maybe I could have the PMF come all the way down to the frame rails then maybe have some type of trim piece like aluminum or something that sandwiched the PMF to the metal frame? That would keep water from being able to wick up to my base layer of 1/2” Baltic Birch. Anyone tried this? Would it work?
Also I’ve countersunk all my wood screws and need to fill in all the screw head holes. What’s the best thing to use besides epoxy”. I just don’t have the time for epoxy and to me it’s just not that easy especially on a vertical wall. What can I use that I can fill these holes with then sand flat?
Any help appreciated. Like I said I’d like to do it right this time. 3rd times a charm.

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Also I’m trying to figure out a different way to do my bottom edge of the PMF. When I built this I should have made it so my bottom of my sides cam down and contacted the metal frame but I have a 1/2” piece of Baltic Birch on the bottom and my sides only come down to the tops of those sheets. I’d like to figure out a way to extend the PMF maybe clear down on my Frame rails. I’m not sure if PMF would stick to metal but I was thinking maybe I could have the PMF come all the way down to the frame rails then maybe have some type of trim piece like aluminum or something that sandwiched the PMF to the metal frame? That would keep water from being able to wick up to my base layer of 1/2” Baltic Birch. Anyone tried this? Would it work?
Also I’ve countersunk all my wood screws and need to fill in all the screw head holes. What’s the best thing to use besides epoxy”. I just don’t have the time for epoxy and to me it’s just not that easy especially on a vertical wall. What can I use that I can fill these holes with then sand flat?
Any help appreciated. Like I said I’d like to do it right this time. 3rd times a charm.

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