Ok, so the part that I initially made seems ok, but feels like it had a bit of tack left to it on the back side. When it is warmed up it loses its rigidity and becomes extremely flexible. What could I have done wrong for it to be that way?

When epoxy is not fully cured the panel could almost sound like a sheet of wax when held up and tapped with your knuckle.
Providing you have mixed the epoxy with the right ratio, if you heat the panel up for a few days at say 100f the panel will cur fine.
perhaps your shop is too cold.
Once cured the tap test will sound like a sheet of tin rather than wax.
Been there done that.
Hope this helps.
Fred 

Thanks for the reply. Will the product eventually cure out, or do you think the heat is necessary. It just seems so weird that it is hard until I put heat to it.

thats normal,

Remember epoxy can take A while to reach full cure also can depend on laminate thickness,most important epoxy has a .TG. The point at which epoxy  will soften then as it cool's it will harden again,post curing the part helps,this time off year can take a long time to cure due to weather.  

Do you guys believe in vaccum bagging even with hand lay up? I would prefer to do vacuum infusion but it just seems like a waste to make the molds for virtually flat panels.

for flat panels and vacuum bagging you dont need a mould, just use a mirror or glass-plate

Matthieu Libeert
Founder MAT2 Composites X Sports
website: www.mat2composites.com