*Why did my remaining Arm-R-seal thicken in the can?

Oil-based topcoats are "oxidizing" products which means that the moment it is introduced to oxygen, a chemical reaction will cause the finish to begin to harden. As topcoat is used, the empty space in the can is filled with air, furthering the curing process. 

Tips to ensure your Arm-R-Seal remains in great working condition:

1. Decant the amount of Arm-R-Seal you will need into a foil covered bowl or paper cup and reseal the can right away. Every minute the can is open, the stain is curing.
2. Be sure to thoroughly clean the chime and the lid of the can to assure a tight seal.
3. Add Bloxygen to remove excess oxygen from your can of finish before sealing. 
4. Store your sealed cans upside down. The liquid creates a seal around the lid when stored upside down, preventing air from getting in. 
5. Transfer any unused finish to a smaller container once finished with your project. StopLoss Bags® are a popular choice.