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Osmosis

What is Osmosis and how is it caused?
When two liquids with different concentrations are separated by a semi-permeable membrane one liquid will penetrate the membrane in an effort to equalise the difference in concentration. During this process the volume of the most concentrated solution will increase and a pressure can develop.

What does this mean to us?
In our case the membrane is the gelcoat of the glassfibre boat and the solution is water. In time water will be absorbed into the GRP. Whilst the majority of this moisture will pass directly through the hull into the bilges with little damage, a certain amount of moisture will eventually start to break down some of the materials used in the laminate itself which will lead to a build up of “osmotic” fluids. Osmotic fluid contains acetic acid and glycol and has a higher molecular weight to water which can make it unable to pass back through the gelcoat or laminate. As more moisture is absorbed through the GRP, an hydraulic effect occurs which eventually leads to blisters being formed in the gelcoat. Most unprotected glassfibre boats at some stage during their life will develop osmotic symptoms to some degree. The timescale and extent of this depends upon various factors including temperature and type of water, periods afloat versus ashore and, most importantly, the quality of the original laminate.

Recognising the problem
The first visual signs of osmosis will be the formation of blisters in the gelcoat.

If you pierce an osmotic blister the liquid contents will have a sour smell similar to vinegar. Osmosis can be detected before any visual symptoms appear by checking the moisture content of the hull with a moisture meter, although this will only give an indication and we recommend that you seek professional advice to determine the degree of the problem and what future action to take. Hempel have Approved Osmosis Treatment Centres throughout the country who are qualified to assist you; contact Hempel for an up to date list.

High Protect
High Protect is a solvent free epoxy developed to provide maximum resistance against moisture ingress on GRP hulls, which reduces the risk of osmosis occurring. High Protect can be applied to new or old hulls where the laminate and gelcoat are suitable. High Protect has an excellent reputation as an anti osmosis treatment and you will find it easy and tolerant to apply under most conditions. It is available in 2 colours to allow application of alternative coats of cream and grey to ensure total coverage of each coat. Compared with alternative products, it has higher build properties, faster overcoating times with a longer overcoating window, a relatively long pot life and is non carcinogenic.

Osmosis Protection

New boats
The initial defence against osmosis is the correct methods and materials used during build. The higher the standard of the GRP laminate at construction stage the greater the defence against osmosis. Applying High Protect will give the ultimate protection to a new hull.

Used boats
Before applying a protective epoxy coating to older boats you need to assess the condition of the laminate and gelcoat. If in doubt we recommend you seek professional advice. If the hull condition is suitable, Osmosis Protection can be carried out using High Protect.

Osmosis Treatment
If the laminate is poor and there are indications of osmosis, a full Osmosis Treatment may be required. Full treatment involves the removal of the gelcoat which, under the correct conditions, will allow the hull to dry out to the recommended levels, before coating the bare laminate with High Protect to give the hull a moisture resistance coating. We recommend that you always employ the services of a professional.

Protection against Osmosis

Preparation for osmosis protection
Correct surface preparation is vital to achieve a longlasting protective finish, so time spent at this stage is well invested.

New Boats
New gelcoat should be cleaned thoroughly with Degreaser to remove mould release agents used in the construction of the hull. After degreasing, the surface should be abraded with 60-80 grade abrasive paper, or lightly slurry blasted, and washed off with fresh water. When dry the hull should have an even matt finish.

Used Boats
Remove all old paint or antifouling by manual dry scraping, using Paint Stripper or having the hull grit or slurry blasted (this is a quick and efficient way of removing the old paint layers and also provides a well keyed surface which will require little extra preparation). Whichever method is used the hull should have an even matt finish with no trace of previous coatings. Freshwater wash the hull and allow to dry. Any minor cracks and blemishes to the hull should be filled after the first coat of High Protect with Epoxy Filler.

HEMPEL HINTS – Osmosis Protection & Treatment
  • If  High Protect application cannot be carried out indoors, it is advisable to erect a cover around the hull to provide protection from the elements.
  • If  High Protect is being applied at 10oC, keep the product indoors at normal room temperature before use.
  • Move the tape line up with each coat to avoid a hard ridge at the waterline.
  • High Protect is produced in 2 colours so you can apply alternative layers of cream and grey to ensure total coverage of each coat.
  • Stir both the base and activator of  High Protect thoroughly before mixing together. Once mixed together, again stir well to achieve an even consistency.
  • Do not thin High Protect.
  • Thorough surface preparation and the careful following of all instructions is the key to a successful finish.

See application of High Protect and specifications.

Osmosis Treatment

Preparation
The gelcoat will need completely removing to allow the hull to dry out.

Hand Power Tools
Grinders provide an inexpensive means of removing the gelcoat and preparing the surface. The disadvantage is the time to complete the task and the amount of dust produced. Protective clothing must be worn by the operator, especially dust mask and eye protection.

Slurry or Grit Blasting
Either of these methods successfully remove the gelcoat and prepare the hull for the application of  High Protect. Any hull defects will become apparent during blasting, ie voids in the laminate. You will need to employ a professional with the correct equipment to carry out either of these methods.

Gelcoat Peeling
A gelcoat peeler planes away the gelcoat at a preset depth resulting in an even and smooth finish. This is generally accepted as being the most effective method for removing gelcoat. Once peeled the surface will require abrading with 40 grit grinding disks, or light slurry blasting to provide a key for the High Protect. This method requires a professional operator.

Peeling a hull prior to osmosis treatment Once the gelcoat has been removed, the hull should be steam cleaned, or at least pressure washed to remove salts and other contaminates which are present in the laminate due to osmosis. The salts and other contaminates will continue to come to the surface so it is necessary to repeatedly wash the hull, we would suggest once a day. On average a hull can take between 4 weeks and 3 months before it is dry enough to recoat. Expert advice at this stage is essential to ensure that the hull is adequately prepared for the application of High Protect.

Any minor cracks and blemishes to the hull should be filled after the first coat of High Protect with Epoxy Filler.

Application of High Protect
From the moment the High Protect base and activator are mixed together the product will start its chemical hardening reaction. Only mix as much as you can apply during the pot life stated on the data sheets. Higher temperatures will decrease the drying time and pot life.

Apply High Protect with a felt roller (you can lay off the surface with a brush if an improved surface finish is needed). High Protect should be applied at a minimum of 200 microns thickness per coat, this can either be measured using a wet film thickness gauge or by working out the surface area of the boat and calculating the correct amount of High Protect needed per coat (1 litre of High Protect will cover 5m2 at 200 microns).

High Protect has a minimum application temperature of 10oC, if possible keep the job at a steady temperature with low humidity, best achieved under cover.
NOTEHigh Protect must not be thinned.

Curing
High Protect should be touch dry within 6 to 8 hours. The instructions will give you a guide to overcoating times which are 8 hours to 5 days at 20oC.
If the temperature drops below 10oHigh Protect is likely to stop curing. The curing will start again once the temperature increases, however an inadequate curing temperature may cause amine sweating to occur. This is a condition where a thin film of amine carbomate forms on the surface of the epoxy and this amine sweat must be degreased or washed off with fresh water before overcoating.

Planning your overcoating times is essential if you want to apply the paint without rubbing down between coats.

Full cure of High Protect will take approximately 7 days at 20oC and it is recommended that the boat is not launched during this period as it could effect the curing of the coating.

Where 10oC cannot be achieved Light Primer may be used instead of High Protect – for more detailed information contact your local Hempel office.

HEMPEL HINTS – Osmosis Protection & Treatment
  • If High Protect application cannot be carried out indoors, it is advisable to erect a cover around the hull to provide protection from the elements.
  • If High Protect is being applied at 10oC, keep the product indoors at normal room temperature before use.
  • Move the tape line up with each coat to avoid a hard ridge at the waterline.
  • High Protect is produced in 2 colours so you can apply alternative layers of cream and grey to ensure total coverage of each coat.
  • Stir both the base and activator of High Protect thoroughly before mixing together. Once mixed together, again stir well to achieve an even consistency.
  • Do not thin High Protect.
  • Thorough surface preparation and the careful following of all instructions is the key to a successful finish.

See application of High Protect and specifications.

Specifications

Osmosis Protection

Product no of coats recoating interval @ 10oC min-max recoating interval @ 20oC min-max coverage m2/ltr wft per coat dft per coat thinner
Epoxy Filler – Filler if needed 18h-54h 8h-24h 845 tool cleaneronly
High Protect 2 18h-11h 8h-24h 5 200 200 845 tool cleaneronly

Tiecoat option 1

Light Primerthinned 5% ontoHigh Protect 1 18h-11d 8h-5d 12 80 40 845
Hempel Antifouling onto Light Primer 2 2h-8h 1h-4h 13 75 40 808

Tiecoat option 2

Underwater Primer onto High Protect 1 18h-27h 18h-12h 9.5 100 40 823
Hempel Antifoulingonto Underwater Primer 2-3 9h-indefinite 5h-indefinite 13 75 40 808

Osmosis Treatment

Product no of coats recoating interval @ 10oC min-max recoating interval @ 20oC min-max coverage m2/ltr wft per coat dft per coat thinner
High Protect 1 18h-11d 8h-5d 5 200 200 845 tool cleaner only
Epoxy Filler 1 18h-11d 8h-5d 845 tool cleaner only
High Protect 3 18h-11d 8h-12h 6 150 150 845 tool cleaner only

Tiecoat option 1

Light Primerthinned 5% ontoHigh Protect 1 18h-11d 8h-5d 12 80 40 845
Hempel Antifouling onto Light Primer 2 2h-8h 1h-4h 13 75 40 808

Tiecoat option 2

Underwater Primer onto High Protect 1 18h-27h 18h-12h 10 100 400 823
Hempel Antifoulingonto Underwater Primer 2-3 9h-indefinite 5h-indefinite 13 75 40 808