It should be fine. No need to spray. Set it up and check out the size of the illuminated patch on the floor, then jusdge what the mounting hight should be to just illuminate the plants your need. You might find that a commercial reflector lets the light spread too much rather than concentrating it on a small area.

I've read that flat white paint reflects 80-90% of the light while aluminum foil only reflects around 50-60%. Is this true? If it is wouldn't my aluminum reflector be about the same as foil?

Sounds like something a paint manufacturer might write :)

A typical new white paint might have a reflectance around 80%. They will also typically degrade with age, very quickly under heat, metal halide lights, or high humidity. You can get paints with reflectance up to 90% or so but they are more expensive and often degrade more quickly. You may also see figures quoted up to 95% or occasionally higher for white paint. These are misleading and will only apply with special application techniques or over a limited range of wavelengths.

Pure Aluminium has a reflectance in the main visual wavelengths of about 95%, and above 90% across the whole visual range. The reflectance drops off rapidly into the UV but remains high in infra-red. Aluminium oxidises to form a fairly transparent film that resists further corrosion. Total reflectance is only reduced by a couple of percent by the oxide layer. In coastal areas, salt can cause the formation of Aluminium Chloride which is a milky colour and reduces reflectance quite a bit. In severe cases, the reflector may corrode badly and need to be replaced. "Shiny" reflectors for lights should generally have a rough surface so that light is reflected more diffusely and not focussed too sharply.

Aluminium foil is quite good at reflecting light, but does tend to be too thin and include small holes. Reflectance is still good in infre-red and red light but not so good for blue light. It is also relatively fragile and hard to keep to a shape.

Aluminium is more expensive than paint. Use Aluminium for a small tough reflector around a lamp. Use white paint for the ceiling, walls, and floor of a growing area.