GftK VDW 850 vs Sika FastFix for Jointing a 40-Square-Metre Patio

GftK VDW 850 is a two-component epoxy-based slurry made to be washed into joints with plenty of water and a soft broom. Sika FastFix is a single-component, moisture-cure polymer compound swept in dry and then activated with a mist of water. Confusing those workflows can spoil a 40 m2 job in one afternoon, because VDW 850 needs a saturated surface and FastFix needs a bone-dry one.

VDW 850 covers roughly 40 to 60 kg across a patio this size at a joint width of 8 mm and a paving depth of 40 mm. Wienerberger clay pavers are usually 50 mm deep in their patio ranges, which pushes consumption toward the top of that band. Sika FastFix is sold in 15 kg pails and covers a smaller area per unit, so the same patio needs several tubs. Underbuying causes trouble with both products, since each can skin or set before a second delivery arrives.

Working VDW 850 wet

VDW 850 starts with soaked paving. After the two components are mixed, the slurry is tipped across the surface and pulled with a squeegee and soft broom diagonally across the joints. The aim is to fill every joint until it sits proud and slightly domed, then wash the paving with a fine spray while brushing away the remaining film. This rinse is where most jobs go wrong. On porous natural stone, especially Indian sandstone and riven limestone, epoxy haze bonds into the surface pores if wash water dries before the residue is fully removed. Dense granite setts and vitrified porcelain carry much less risk because there are no open pores for the resin to key into.

That milky film is the reason people reach for Lithofin MN sandstone cleaner a week after the job. The cleaner works, although it is an acidic-leaning intensive cleaner, and repeated use on soft sandstone opens the surface further. A cleaner finish comes from keeping three buckets of clean water ready for a 40 m2 pour and changing the wash water as soon as it turns cloudy.

At 15 to 20 C, VDW 850 remains workable for roughly 30 to 45 minutes. In direct summer sun, a dark clay paver can reach 40 C surface temperature by mid-afternoon, and the working window shrinks fast. Smaller batches, shade over the working area, and a start on the side of the patio the sun has already left all slow the skinning. A tarpaulin over the active section can add another quarter hour.

VDW 850 stays water-permeable after curing, so it suits a patio laid over a free-draining bedding layer. On a solid mortar bed with no falls, the joints hold standing water, and a north-facing patio then gives black algae the damp surface that no jointing compound resists. Drainage, more than anything, decides how the finished joint behaves.

FastFix and the dry-sweep mistake

FastFix needs paving that is dry to the touch. The compound is swept into the joints, compacted with a stiff brush, cleared from the surface, and then misted to trigger curing. If too much water lands on the paving before the excess compound has been brushed away, the polymer lifts into a smear that dries hard and grey across the paver face. On a textured clay paver, that smear catches in the sanded surface and will not brush away once set.

The compound suits joints from 3 mm upward and cures firm enough to resist weed growth and ant excavation. Those two failures often finish kiln dried sand block paving within a couple of seasons. Kiln dried sand is cheaper by an order of magnitude, although it washes out of any joint wider than 5 mm and disappears from patios that get pressure-washed. FastFix holds in joints where sand walks away.

Cure speed is one of its main advantages. FastFix firms within a few hours and takes light foot traffic overnight, while VDW 850 needs 24 to 48 hours before walking on it. On a job that has to be handed back quickly, that difference can decide the product.

FastFix is less tolerant of damp at the bottom of the joint. If water sits there because the bedding layer never drained, the cure turns patchy and soft crumbly sections appear. Those weak areas often fail during the following winter.

Sub-base, bedding, and clay pavers

A domestic patio carrying foot traffic and garden furniture normally uses a Type 1 MOT sub-base of 100 mm, compacted in two 50 mm layers. A single 75 mm lift leaves the bottom poorly compacted, so the bed settles unevenly and joints crack open whatever compound was used. Each layer should be compacted with a plate wobbler until the aggregate stops moving under the plate, usually after four to six passes.

A 30 mm sharp sand and cement screed on top of that sub-base creates a rigid bed and holds falls accurately, but it traps water. VDW 850 can cope with that build only when the patio has a crossfall of at least 1 in 80 toward a drainage channel. A free-draining grit bed lets water pass through permeable joints and move away, which is the logic behind pairing an epoxy slurry with a drainage build instead of an impermeable mortar tray.

Wienerberger clay pavers alter the laying routine. They are fired to a consistent thickness, usually 50 mm for patio ranges, so the bed can be screeded flat with confidence. Concrete block paving varies more in depth from batch to batch, which is why installers moving between the two materials tend to use a longer straightedge for clay and check every fifth unit against a string line.

Material cost over service life

VDW 850 for a 40 m2 patio costs several times more in materials than kiln dried sand, before wash water and the soft brooms it wears out are counted. Sika FastFix sits between those two on material price. The higher initial cost is partly offset by the longer service life of a joint that resists washout, weeds, and ant excavation.

Efflorescence after laying

A white bloom across new paving three weeks after laying is efflorescence: soluble salts drawn to the surface as the bed dries. It is not a jointing failure, and no compound prevents it, because the salts come from cement in the bedding or from the paver body itself. Concrete block paving effloresces far more than fired clay, which is one quiet argument for Wienerberger clay on a patio expected to look settled by the first summer.

Rain usually weathers the bloom away over a few months, and a dedicated efflorescence remover speeds the process. Strong acid on fresh sandstone can burn the surface and etch a permanent dull patch. On clay pavers, a mild proprietary remover followed by a thorough rinse deals with the bloom without damaging the surface. Pressure-washing it hard treats the bloom like dirt, drives water back into the bed, and can pull up another crop of salts a week later.

With VDW 850 the diagnosis gets harder, because epoxy haze from a rushed wash and genuine efflorescence can both look like the same white film. The test is simple: efflorescence dissolves and lifts with water and a mild remover, while cured epoxy haze will not move without Lithofin MN cleaner and a stiff pad. A salt remover will do little against cured epoxy residue.

Resin bound paths beside the patio

A garden path feeding into the patio does not have to use jointed paving. A permeable resin bound path uses UV-stable resin mixed with dried aggregate and trowelled to a seamless finish. It drains through its own body and has no joints to fill.

That surface costs more per square metre laid than block paving jointed with FastFix. It also needs a rigid base of open-graded asphalt or concrete, not a flexible sand bed. In exchange, the path removes the joint as a maintenance item altogether.

Where the two approaches part company is repair. A block patio jointed with VDW 850 lets a cracked paver be lifted, reset, and re-slurried around its four joints. A resin bound path cracks as a sheet and patches visibly, and that visible patch is what you accept for a surface with no joints to weed.