Standard Bathtubs
Joy of St Croix - Tub Repair and More refinishes
standard residential built-in bathtubs made of
porcelain, acrylic, or fiberglass
"Standard", meaning: the accepted pattern
The most common type of bathtub installed in residential homes is a five foot wide (left to right), two and a half foot deep (apron to rear wall) bathtub with a single apron installed into a framed recess usually located along the exterior walls for the main level or by a load bearing wall on the second or third level.
Please, note that the bathtub is installed during the framing stage of construction before the drywall walls and ceilings, or the flooring, or the etc., etc. 

What is a bathtub's "apron" or "skirt"?
When bathtubs were first introduced they were free standing and were treated as furniture.  Since there was no drain anchoring it in one position they could be placed wherever it was convenient, and moved about to clean underneath or behind them. Once plumbing was introduced the bathtub couldn't be moved and dirt and dust would collect underneath and behind it. 
Fast forward to the modern home building era. Bathtubs could now be made from thin, light steel saving costs on material, shipping, and installation labor (one guy vs four to lift and manoeuvre). Architects wanted clean lines and no wasted space. The simple solution was to extend the rolled rim of the clawfoot bathtub to the walls behind and to the sides, and then extend the front rim to the floor...like an apron.


apron: single

Paint can't adhere to your tub if something's in the way...like a previous reglazing job. Do you expect the old caulking to be removed and replaced with new, or just painted over?
apron: double

Paint is weakest at its edges. In a bathtub that's the drain, the overflow, and the perimeter caulking line. Do you expect your drain to be painted over?
apron: triple

After sanding and polishing, the vulnerable edges of the paint need to be sealed. For the money you're spending, what quality of drain, overflow cover, and caulking do you expect to accept?
Typography is the art and technique
When bathtubs were first introduced they were free-standing tubs and were treated as furniture. Since there was no plumbing they could be placed wherever it was convenient, and moved about to clean the room. Once plumbing was introduced the bathtub couldn't be moved and dirt and dust would collect underneath and behind it.

