My thought is that if it's going to happen it's going to happen. I don't think that adding sealant before it leaks is going to prevent anything. There have been many theories bandied about on these threads as to what is causing the evap problems- from corrosive r134a creating holes from the inside to pollen or copper particles from the blower motor creating corrosion from the outside. I bought into the pollen theory so I put in a pollen filter. My compressor was cycling rapidly last summer so I recharged the A/C. If it doesn't hold then I will consider a sealant and add dye to see where the leak is. It's not always the evap. It could be an o-ring or some other union that is leaking. Good luck.