"That looks definately like hot burn, no question about it."

Yeah but you're a stoner. What could you possibly know?

If we were all close I'd bet you both dinner that it wasn't the leaves being burnt because of that masonite 'absorbing' enough heat to do that, but alas we aren't. Best thing I guess it to either change it, or keep the 'leafs' away from it. Then it's what I call, a non issue.

As for the ferts and leaf tips burning? In the world famous overgrow.com's Fertilizing & feeding : Nutrient deficiencies it says this:

"Tips of leaves are yellow, brown, or dead. Plant otherwise looks healthy & green. Stems may be soft >> Over-fertilization (especially N), over-watering, damaged roots, or insufficient soil aeration (use more sand or perlite. Occasionally due to not enough N, P, or K"

So you can sort of go from there and try to figure out what caused it. If you say it hasn't had much N then that's taken out of the equation.