Polycarbonate lenses have always presented a challenge for those who wish to tint them. With lenses made from CR-39® monomer plastic and BPI® Molecular Catalytic™ tints the tinting process is much more straight forward. These tints use a carbon molecular that shares an electron with the carbon molecule in the plastic, so that the tint bonds to, and becomes part of the lens structure itself. The end result is that the lens cures in the tinting process and has a more scratch-resistant surface. Polycarbonate, unfortunately, is very resistant to tints. To tint polycarbonate lenses one must tint the scratch coating and the better the scratch coating the harder it is to tint. The first type of coatings that were applied to polycarbonate were so resistant to tints that obtaining a sunglass shade was almost, if not, impossible.
In recent years, the types of coatings that are used on polycarbonate lenses have changed considerably. It is much more common today to see a combination of coatings used on polycarbonate. Typically, the front surface, which tends to receive the most scratches, has been very effectively coated by the factory. The back surface is now commonly coated by the optical lab. The coating on the back surface is definitely tintable, even to relatively dark sunglass shades.
There are some fundamental principles for tinting polycarbonate that will help you considerably. (Many of these ideas apply equally well to tinting plastic lenses).

Before tinting:
Ensure that both lenses are from the same source, preferably the same batch number. Polycarbonate and lenses made from CR-39® monomer both vary significantly from different manufacturers and from manufacturer’s batches.
Clean the lenses with BPI® Lens Prep II™. This conditions the surface of the lens and reduces the surface tension of the scratch coating.
Check your lens tinting instrument thoroughly. Make sure that it is reaching and maintaining a steady temperature. Check the heat transfer fluid. If it looks old, replace it. Always use a quality product like BPI® heat transfer fluid. Use of other substances is not recommended and can be hazardous to your health. Ensure that the tint tank is immersed sufficiently into the heat transfer fluid in accordance with the manual of your lens tinting instrument. Mix the tints with distilled water. This will eliminate the possibility of any mineral contamination that may exist in your local water supply.
Mix and stir the tints well. Also, continue to stir the tints at regular intervals. If you notice patches of unevenness (blotches) in the tinted lenses, it is time to change the tint.
Check the temperature of your tint bath with a quality laboratory thermometer to ensure that the operating temperature of the tints is between 205°F and 210°F. Polycarbonate (or lenses made from CR-39® monomer) will not tint properly at even slightly cooler temperatures. Finally, the coatings of polycarbonate lenses will absorb moisture from the air. This can create difficulty for the coatings to absorb tints. Reduce the exposure of the lenses to high levels of humidity (For instance, around a steaming tint bath) prior to tinting.

During tinting:
Determine for yourself a standard for the particular tint that you are using. Use a sample lens with a freshly mixed batch of tint. Ensure that all the conditions above have been fulfilled. Tint the lens for 15 minutes. Rinse dry and set aside. Test future lenses that you tint with that color against the standard after 15 minutes. When the tint cease to match the standard, change the tint.
A common mistake is trying to get more lenses from each batch of tint than is practical. It is much less expensive and time consuming to change the regularly than it is to keep pushing the numbers of lenses out of every batch.
Limit the amount of time that the lens spends in the tint or neutralizer. You should not exceed a half hour. If you are typically spending longer, then you should change your tint.

Neutralizing:
If you need to neutralize your polycarbonate lenses, use a top quality water-based neutralizer like BPI® H20 Neutralizer™. Keep the temperature of the neutralizer below 210°F and do not exceed about ten minutes in hot neutralizer. Incorrect neutralizing can cause crazing of the lens.