Newbie here renovating a trailer

[RickLincs]

 

[Burnerman]

Try centre punching the rivet centre prior to drilling it out.....if it's very hard the head may need to be carefully ground off. can you access the other side?
Returning to your original query re the plywood, I've used both the phenolic and exterior grade before. To protect the underside when using exterior grade timber I've used a layer of activated fibreglass resin for weather protection, and when sprinkled with sand gives good grip for animal feet.
John

 

[RickLincs]

Cheers Burnerman.
Ive actualy visited a local coach works who can supply the boards for the sides, and said they will do me a good price if I dont mind having slight seconds.
I suspect I'm going to have to get some quality bits to drill these out as mine don't seem to be making much impression on them. Picture of the inside.

 

[Burnerman]

For sure, some quality HSS or carbide tipped drills will give you every chance, but you could try grinding the head off with an angle grinder if you have one and access is ok.
If the rivet head actually punched (rather than the punch just bouncing off) then the metal can't be too hard.
John

 

[RickLincs]

Yes, it can be dot punched. I'm wary of using the grinder as the head is a tight fit upto the aluminium trim

 

[opps]

Personally, I would use a TCT drill bit.

https://www.ebay.co.uk/itm/350778112764

They are suitable for stainless steel. That said, I suspect that your rivets aren't SS given that you can punch them with ease. I find them more forgiving than HSS carbide tipped when using the wrong speed or no lubricant.