I have a few suggestions, for what they are worth:
Exercises that strengthen the abdominal muscle are what give support to the back...if you have poor abdominal tone, you are more prone to lower back problems. The physical therapy department of you local hospital may be able to give you input on which exercises could be most helpful as well as material on good body mechanics although they might want a referral from your physician before they do so. This could be very worthwhile though if it prevents significant back injuries that could be incapacitating for weeks or months.
Proper body mechanics will also help reduce creating back pain. You need to keep you back straight when working and not over-extend your arms while holding heavier pieces of equipment such as the polisher. Bend from the hips, keeping your back straight to apply pressure ot the polisher and remember to let the polisher and not excessive pressure do the work. Use the polisher with your elbows roughly in line with your torso. If you move your arms forward, thus moving the elbows away from the body, you are using only your arm and upper back muscles to support the equipment and apply desired pressure. Keeping you elbows at you side and moving your entire body forward (with as little bending as possible) keeps the back in better alignment and allows a more equal support from the muscles of the entire torso. Position yourself closer to the surface you are working on rather than extending your arms. Consider using the stepladder more often (I have mine wrapped with some layers of that foam shelf liner stuff so that I can get it right next to the vehicle without marring the finish) One of those platform type steps would be very useful. Remember to get high enough to work without extending you arms too far and bend from the hips with a straight back for working on the center of the hood, roof and upper portions of vehicles such as SUV's. For work on the lower panels, I use a foam kneeling pad tha I have for garden work and use a kneeling position. I have also just sat on the pad to do wheels. Remember to keep your back straight. The trickiest part in doing these maneuvers is not to curl your back forward to see what you are doing. It would be very useful to drive the vehicle onto ramps that elevate the vehicle a foot or so off the ground. Avoid any work in a bent or squatting position, especially if you are doing things that require applying pressure....like scrubbing wheelwells and rocker panels. Get on your knees or sit.
Take breaks of a couple minutes frequently standing staight without holding the weight of the polisher. Use the proper technique for holding the polsiher as recommended by the maunufacturer (that's part of why they make them....to minimize fatigue)
The literature has been variable about the effectiveness of back support belts. You may have seen people who lift alot with those wide black belts. Some people swear by them. I think their most immediate usefulness is in restricting the bad habit of bending at the waist (thus curving your back) instead of bending from the hips that keeps the back straight. In other words...it can be an effective reminder to use proper body mechanics. The also provide some support to the lower abdominal wall until these muscles have improved strength from your exercises.
When lifting, keep you back straight and the object close to your body and use your legs to stand. You may think the weight of polisher is not all that much but it actually can be a significant weight load on you upper body. To get an idea of just how much the weight of the polisher can cause muscle fatigue, try holding it in your hand with your wrist straight and then with your wrist bent away from you and note how much more tiring it is to hold the polisher with your wrist bent. You will understand very quickly why it is much better to use good body alignment when working. Your head weighs about ten pounds as well. The same strain and muscle fatigue will be generated in your neck and upper back if you don't keep your head up and neck straight while working.
It can take a bit of creative thinking to develop ways of working that facilitate good body mechanics and investing in some of the things like ramps, platforms and support belts will cost money. They are a good investment if they prevent serious back injury. Nothing replaces the value of exercise to strengthen abdominal muscles. Weight loss, if over weight is of huge benefit as well. Simply walking briskly with good posture is good exercise and will increase total body fitness as well as helping to lose weight and it costs nothing.
These suggestions are all things that I have gleaned from physical therapists and practiced myself over the past 30 years in Nursing. I move and help lift patients that can weigh over 300 pounds and have never had a back injury. I hope theyare of some help to you and other members.