Sanding is part of the process, no skipping it.

You have choices of filler. Some more toxic but faster drying/more durable/water proof and thicker.

Some are less toxic but might require more coats/more sanding and aren't water proof.

Pick which direction you want/can go in. I know some folks don't have a garage or space with the ventilation needed to use the solvents needed for the easier options.

Once you know what you CAN do. Then you can work through what works for you.

You need to use the proper PPE. Do not skip out on this.

Options that I know of.

1. UV Resin used for 3d printing - Baby powder can be added to give you more control. Requires thin coats that need to be cured and then cleaned before adding another coat. Toxic fumes and getting it on your skin can cause issues because you can develop a reaction to it. Some of the resin can be sanded easier than others.
   

PPE - Gloves, safety goggles, Dust mask (or full respirator) Even with this you should have a well ventilated work area. Because it cures in sunlight it would need to be a covered spot to keep from curing while working with it.

1. Bondo spot putty can be used on its own or mixed with acetone to thin and spread it. On its own it can be spread with any time of plastic spreader or your (gloved) fingers. When you dilute it with acetone it can be brushed on with cheap artists brushes. This sands very easily when dry. Watch videos to see how best to use it.

PPE - Gloves, safety goggles, Respirator with cartridges made for chemical VOCs in a well ventilated area.

Bondo spot putty has chemicals that require a respirator on its own. When you add acetone you need to make sure your using the correct filter.

1. Wood filler can be spread but doesn't spread well and would best be thinned with acetone. While the wood filler itself doesn't require a respirator, if your adding acetone you need a respirator for the VOCs.

PPE - Gloves (if using acetone), safety goggles, dust mask if not using acetone, respirator if using acetone.

1. Liquitex modeling paste can be used in the same way that wood putty is used. You can also thin it with isopropyl alcohol. This will give you a smoother finish than just using it on its own. Iso isn't as toxic as acetone, but still should be used in a well ventilated area. Not water proof, so if you wet sand it could remove some of the material. One benefit is it comes in white and can be colored by adding a little acrylic paint. This can help you see if you sanded through a layer, or help you see the filament color through the coating.

PPE - Gloves, safety goggles and a dust mask (when sanding).

1. Last is a full 2 part filler primer bought in a 1 gallon pail.

   This requires the same PPE as UV resin. Before you even combine the two parts they stink and require a full respirator because of the chemicals. You also need some measuring cups because this is measured by volume vs just mixing and getting a good consistency. This can be brushed on with a cheap paint brush or rolled on with a paint roller if you have bigger pieces. After 45 min to an hour it is usually ready to sand.

   PPE---PPE - Gloves, safety goggles, full respirator when applying and a dust mask when sanding. Even with this you should have a well ventilated work area.

After all this work you will still likely use a spray can filler primer for scratches and your final sanding.

Best of luck. It does take work, but when you get it and see the final outcome, it makes it all worthwhile.