Is sclerotherapy for seroma painful?

the sclerosing involved with treatment of a seroma is not referred to as sclerotherapy, the latter normally referring to the cannulation of a vein with a very tiny needle. This type of treatment is performed rarely for such a complication after liposuction and other surgical treatments.  Both treatments to involve injecting a caustic agent that creates inflammation with an end point of damage to the tissue such that the body walls off the opening.  Sclerosing of the cavity of a seroma is usually suggested by the surgeon if the drain and compression doesn't help. It can be slightly to moderately painful but can avoid further surgery which is not guaranteed to work.
 

You are confusing sclerotherapy for veins with sclerotherapy for cysts, etc. Sclerosants are different for each problem and it depends what the seroma is from - post surgical, lymphatic, based in the spinal canal (hopefully not) and other. Talk to the doctor who is treating this 'seroma'

If infection is controlled, Complete Decongestive Therapy (CDT) is the least invasive treatment. CDT is the safe, effective use of manual techniques and bandaging to control swelling and edema. If you are highly motivated to learn about these techniques, look for a Certified Lymphatic Therapist (CLT). The National Lymphadema Network (NLN) and Lymphatic Association of North America are good places to start. These techniques have only been introduced to the United States recently. Although very few Physicians and Therapists have this type of experience, we have successfully treated back seromas following complex spine fusion surgery for Scoliosis and following Thoracotomy.

A seroma is a pocket of fluid that sometimes forms post surgical procedure. Sclerotherapy is most effective and preferred for spider veins. Seroma would require a different treatment course.

I would recommend surgical excision of the lymphocele (seroma) because this collection of fluid is due to a disruption in the lymphatic fluid flow.  This is an uncommon problem patients can also experience after having an ambulatory phlebectomy performed for varicose veins.  The removal of this lesion is pretty simple and not very invasive. Any experienced surgeon should be able to take care of it without much fuss.