Decorator's Prep Calculator — Sugar Soap, Filler, Caulk & Sandpaper

Concentrated liquid sugar soap is heavily diluted before use. Bartoline's data sheet specifies about 125 ml of concentrate per 4.5 litres of warm water for surface preparation, so a single 1 litre bottle of concentrate makes roughly 36 litres of working solution. Applied at around 0.1 L of diluted solution per m², that one bottle cleans several hundred square metres — so most rooms need just one bottle. This calculator works out the diluted solution from your area, then back-calculates the concentrate using the 125 ml : 4.5 L ratio. Always check your own product's label, as dilution rates vary between brands (Screwfix's guide quotes roughly 50 ml per 4 L).

These are estimating rules of thumb for spot-filling cracks and holes, not a manufacturer coverage figure. As a guide, good condition walls need about 0.02 kg/m² (a few nail holes), fair condition with some cracks about 0.08 kg/m², and poor condition with extensive cracking 0.20 kg/m² or more. These spot-fill rates are far lighter than full-surface skimming — for reference, Polycell Easy Sand Interior Polyfilla covers roughly 1 m² per kg at a 5 mm full skim. Use a fine surface filler for small imperfections and a deep-fill product for holes over 5 mm.

A standard 310ml cartridge of decorator's caulk covers approximately 12 linear metres with a 5mm bead. For wider gaps (up to 10mm), coverage drops to about 6 metres per cartridge. Use decorator's caulk (not silicone) for gaps between walls and woodwork — it's paintable and flexible. Apply with a caulk gun and smooth with a wet finger for a neat finish.

Use 80 grit for heavy sanding and paint removal. Use 120 grit for general smoothing of filler and between coats. Use 240 grit for fine finishing between topcoats. Each sheet of 120 grit covers roughly 2 m² before it wears out. For large areas, a sanding pole or orbital sander saves significant time. Always sand in the direction of the wood grain on timber.

Light sanding between coats with 240 grit gives a smoother finish, especially for gloss and satin finishes. It removes dust nibs and provides a key for the next coat. For matt emulsion on walls, sanding between coats is usually unnecessary unless there are visible imperfections. For best results on woodwork, lightly sand and wipe with a tack cloth between every coat.