Prairie 2-6-2 Question

[Mr Overalls]

Hello, new here to the forum. I recently purchased the Rail Chief set to put up for the Grandson when he comes for Christmas. I have a 4x8 oval set up with a 4% grade on one end with the Bachmann Graduated pier set. The diesel pulls the grade with its full load of cars pretty well.

I just purchased a 2-6-2 Prairie Southern and it will barely pull 3 cars up the grade without major wheel spin. I have added some weight without much success.

I have read about the bull frog snot or plastikote you can add to the wheels for traction.
The Prairie 2-6-2 has 3 sets of wheels. I've put power to each set and it will run from all 3 i believe because they are all connected with metal wheel drive connect rods.

Which set of wheels to add the snot and which to not?

And is this the best idea or recommendation's?

Thanks!