There are a few things that could cause Boracare to leave residue once the application has dried. One possibility is that the solution was not mixed completely before it was applied. Boracare is best mixed in a separate container such as a 5 gallon bucket and using an impeller-type mixer such as one you would use to mix paint. If Boracare is applied very heavily (to the point of runoff), then there can be excess product which the wood cannot absorb, and that could dry in a whitish finish on the surface of the wood. Next, when you have very old wood there could be little to no moisture left in the wood, and that moisture is how the Boracare penetrates through the wood. So this really would go back to just using more Boracare than the wood can absorb. Finally, if there were any kind of finish on the wood such as paint, stain, etc., then this would have to be removed completely before you could apply Boracare or other borate products.
You can use warm soapy water and a scrub brush to remove any excess product that may leave residue on the wood.
Yes, Bora-Care can be applied either by spraying or painting with a brush onto the unpainted/raw wood surface.
Bora-Care is safe to use when yes, used as directed on the product label. It is a green product Unless you physically drinking the solution or ingesting the treated wood they would be no harm, it is a borate based product so slight contact with skin would not be absorbed or harmful and washing off with soap and water when contact is made would be recommended and wearing safety gear to prevent contact.