starting on the Concrete base for the Invader 30cm x 28cm and a much smaller base for the "Super High Tech Jet Fighter" Saab JAS 39 Gripen my previous project.
first I smeared a thin coat of car body filler, let that dry.
View attachment 580858
then when that was dry, sanded out the larger faults and rough edges to produce a nice concrete surface, with a knife line down the centre for the expansion joint, I dunno, does concrete expand in the sun? Maybe just a concrete join then.
My experience as SATCO at an RAF airfield having large-scale groundworks done.
Concrete can crack when it dries due to contraction, When laid for aircraft servicing platforms, it is laid in a deep (ours were ~4m), single strip ~5m wide to ensure that it lays in one homogenous piece. Then the surface is scored at intervals as it dries to ensure that it cracks in the right place, effectively turning the laid strip into large blocks. The gaps are then filled with flexible sealant.
A civil engineer will be along shortly to fill in the details.