I use this stuff, available from Walmart: Bondo glazing and spot putty.
https://www.walmart.com/ip/Bondo-Glazing-and-Spot-Putty-00907ES-4-5-oz-1-Tube/16927984
I used to coat the entire tube to get the small dings and defects in the tube, and it worked well. BUT it was a ton of sanding. I now just use a gloved finger and put it on the grooves, and sand that down, using a second coat to ensure all of the areas where it shrank are filled in. I also use it for filling in balsa grain, after using some Duplicolor filler primer. Best way to do that is to use some lacquer thinner. I use a 3" or so bead on a piece of plastic, and use 1.5-2 standard eyedroppers full of lacquer thinner, mix well and use quickly. The thinner dries fast so make small batches and work fast, using a credit card to apply and scrape off as much as you can.
https://www.walmart.com/ip/Bondo-Glazing-and-Spot-Putty-00907ES-4-5-oz-1-Tube/16927984
I used to coat the entire tube to get the small dings and defects in the tube, and it worked well. BUT it was a ton of sanding. I now just use a gloved finger and put it on the grooves, and sand that down, using a second coat to ensure all of the areas where it shrank are filled in. I also use it for filling in balsa grain, after using some Duplicolor filler primer. Best way to do that is to use some lacquer thinner. I use a 3" or so bead on a piece of plastic, and use 1.5-2 standard eyedroppers full of lacquer thinner, mix well and use quickly. The thinner dries fast so make small batches and work fast, using a credit card to apply and scrape off as much as you can.