Knauf UB888 jointing compound
Premium Jointing powder, also sold as USG Boral / “Boral powder”. The embed and body coats on fibre cement joints.
2 parts powder : 1 part water, by volume. Water into the pail first, then powder.
| Step | Number |
|---|
| Mix ratio | 2 powder : 1 water, by volume — half a glass of powder to a quarter glass of water |
| Order | Water first, then powder. Slake before mixing, then mix as little as possible |
| Set time | ~45 min on paper |
| Real working window | 25–35 min in Tarlac heat |
| Placement per batch | ~15–20 min, then waste the stiff tail |
Slaking — the step that buys the window
Slaking is letting the powder sit in the water untouched so it soaks itself, before stirring at all. Same word as slaking a thirst: the powder drinks.
Dry powder does not wet instantly. Drop it in and the outside of every clump wets while the inside stays dry. Stir straight away and you are forcing water in mechanically, which takes far more stirring — and stirring is what accelerates the set. Wait instead, and capillary action does the same job for free.
- Water in the container first.
- Powder in on top. Do not touch it.
- Wait 2–3 minutes. The mound darkens as water wicks up through it.
- Ready when no dry pale islands are left on the surface.
- Now mix — it comes together smooth in a fraction of the stirring.
- Skipping the slake is what causes the short window. You get lumps, you stir harder to chase them, and the extra stirring collapses the set time. Those two or three minutes of standing around are the cheapest working time you will ever buy.
“Skim” — pin the word before using it
In these notes, skim means DV-5000 over the joint bands. Not the whole wall, and not any other product.
| Usage | Scope | Where it applies |
|---|
| Ours | The joint bands and screw heads only — the same 300–500 mm width as the Knauf coats beneath | Everything on this page |
| General trade | A thin coat over an entire surface to make it uniform | What most guides and videos mean. Do not assume ours |
- The only whole-wall step in the joint stack is the primer — spot-prime the joints and cut edges early, full-prime everything before paint.
- Whole-wall skimming solves one thing: joint banding. Board face and joint band differ in texture and porosity, so under a sheen the bands can telegraph as faint stripes — the same flashing mechanism as an unprimed filler spot. Semi-gloss is the worst case. If the light rakes hard across a wall, that is when the whole-wall version earns its cost.
Recoat gates — set is not dry
Set means the reaction has finished and it is hard. Dry means the water has actually left. They are hours apart.
| Transition | Gate | Wait |
|---|
| UB888 → UB888 | Set — fingernail-hard | ~45–60 min |
| Last UB888 → DV-5000 | Dry | overnight |
| DV-5000 → DV-5000 | Full air-dry | between each |
How to test each gate
| State | Test | What it means |
|---|
| Plastic | The knife still moves it | Too soon for anything |
| Cheese-hard | Holds its shape, but a blade lifts it off in clean curls | Scrape now. This is the knock-down window |
| Fingernail-hard = set | Press a thumbnail in firmly. No dent, no smear — the nail skids and leaves at most a shiny scratch | Recoat now. Still cool and dark is fine — that is damp, not unset |
| Dry | Colour, not hardness. Uniform chalky white, no dark patches, no longer cool to the touch | Only now can DV-5000 or primer go on |
- Two different tests for two different gates. Fingernail tells you it is set. Colour tells you it is dry. Never use one for the other.
- Test on an offcut of the feather edge, not in the middle of a finished joint.
- Gypsum warms as it sets, then cools. A batch that has gone cool again has finished reacting.
- All three Knauf coats fit in one day, because coat-to-coat only needs set. The calendar days in a joint job are material transitions — Knauf to Davies, Davies to sand, primer to paint — not coats.
- The Davies skim is optional indoors. The nook was finished on Knauf alone — sanded, primed and topcoated with no skim, and it held. Skip it and the overnight dry gate goes with it. Verified by doing it
- Knauf sands beautifully. The old advice that setting compounds sand hard, so the last coat must be Davies, does not hold for UB888. It cuts easily — but it throws a lot of dust
- That dust is the catch, and it is the whole reason to care. Skipping the skim does not save you work, it moves the work into dust removal. The skim’s real job was burying the dust-generating surface, not sandability. Vacuum, damp cloth wrung hard, then check in raking light before the primer tin is opened
- Coat 3 is conditional. Skip any joint that already sits flat under the 350 blade. Streaky but flat is finished.
- Do not sand between Knauf coats. Scrape at cheese-hard instead. One light 120 pass before the skim, and that is all.
- Two different dusts, two different hazards. Gypsum joint dust is calcium sulfate — a nuisance dust, so wear a mask but do not panic. Crystalline silica is the fibre cement board itself, which is why machine-sanding across feather edges that run to zero on bare board is the thing to avoid. “Only a bit dusty” describes the gypsum, not the board
- Dust on set gypsum is the peel risk, not moisture. Sanding dust is chemically identical to the joint, so the surface looks clean while carrying a loose powder layer — primer wets it to slurry and roller shear lifts it. Vacuum, then damp cloth wrung hard, and never re-roll a section that has started to tack.
Why the window is short, and what widens it
The short window is the price of not shrinking. Setting-type compounds react chemically, so they can go on thick and hold their volume. Air-dry compounds stay workable forever and shrink. There is no product that does both.
Gypsum crystals nucleate and then interlock all at once, so it goes from plastic to rigid quickly rather than tapering off. That part is chemistry, not technique. These three lower it, and none of them cost anything:
| Lever | Why it works |
|---|
| Mix less after slaking | Agitation multiplies crystal nuclei and accelerates the set hard. The biggest single lever |
| Cold water | Set rate is strongly temperature-driven. Chill a bottle — tap water here works against you |
| Spread thin, wide pan | The reaction is exothermic and self-feeding. A deep blob cooks itself; a thin layer sheds heat |
- More water sets slower, less water sets faster and harder. So mixing stiffer does not buy window — it shortens the reaction. Stay at 2:1 and spend the effort on temperature and stirring.
- Never re-temper a stiffening batch — and now the reason: adding water genuinely would buy set time, which is exactly what makes it tempting. But by then the crystals have begun interlocking, and stirring water back in shears that structure apart. You get the working time back and a soft, weak, porous joint with it.
- A window that collapses to ~10 min is contamination, not ratio. Set gypsum in the working pan seeds a flash set. Use flexible plastic food containers, two rotating, rinsed between batches — the working pan matters more than the mixer.
- Small batches are the right instinct, but each one restarts the clock and wastes a tail. A moderately bigger batch spread thin in a wide pan often nets more mud on the wall than three tiny ones.
Coat widths on square-edge joints
- Flat joints: 300 → 400 → 500 mm
- Outside corner: 150 → 200 → 250 mm per side
- High must be removed, low gets filled by the next material. A knife bridges a valley — it cannot fill one. Never chase valleys, never a coat 4.
Knauf Philippines official video, exact product Verified · working window, contamination diagnosis and recoat gates from own site data, 2026-07 · set chemistry Assumed
Concrete mix classes and strength
Cement : sand : gravel, by volume. Philippine class designations as used in general practice.
| Class | Mix c:s:g | Strength | Typical use |
|---|
| AA | 1 : 1.5 : 3 | ~4,000 psi · 27.6 MPa | Water-retaining, retaining walls, heavy structural |
| A | 1 : 2 : 4 | ~3,000 psi · 20.7 MPa | General structural — beams, columns, suspended slabs |
| B | 1 : 2.5 : 5 | ~2,500 psi · 17.2 MPa | Footings, mass concrete, non-critical |
| C | 1 : 3 : 6 | ~2,000 psi · 13.8 MPa | Lean mix, blinding, fill — non-structural |
The ratio is a ceiling, not a promise. Water is what actually loses you the strength — a mix slackened on site to make it workable can drop a whole class.
- Water-to-cement 0.45–0.60 by weight. Lower is stronger and harder to place; that tension is the whole game.
- Cement per m³ Assumed, 40 kg bags: Class AA ~12 · Class A ~9 · Class B ~7.5 · Class C ~6.
- Strength quoted at 28 days. Keep it damp — concrete cured wet for 7 days is dramatically stronger than concrete left to dry out.
- Do not size a structural pour from a cheat sheet. Anything carrying load gets an engineer and a real spec. This card is for judging what a delivery or a helper is actually mixing.
Mortar and plaster
| Job | Cement : sand |
|---|
| CHB laying | 1 : 3 to 1 : 4 |
| Plaster, scratch coat | 1 : 3 |
| Plaster, finish coat | 1 : 4 |
| Skim / neat finish | 1 : 1 to neat cement |
Standard Philippine class designations and trade practice Assumed — confirm against the DPWH Blue Book or the bag before anything structural