I would avoid the aggression of sandpaper and stick to Scotchbrite as Jim suggests.
Once the paint is hard and you are sure you are not going to do any more paint - fog the inside with Corrosion-X to prevent internal rot - and while the aileron is off it is much easier to get at the inside of the back of the wing through the lightening holes to inspect and fog the wing aft of the main spar.
If you are waiting for your mechanic.
And not planning on painting the wing anytime soon.
